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INTERIM REPORT NO. 4
CONTINUING DESIGN WORK FOR THE
SIERRA COOPERATIVE PILOT PROJECT
REPORT NO. SLWM-81-3
PREPARED FOR
THE UNITED STATES DEPARTMENT OF INTERIOR
BUREAU OF RECLAMATION
OFFICE_OF ATMOSPHERIC RESOURCES RESEARCH
CONTRACT NUMBER 7-07-83-V0008
BY
R.D. ELLIOTT AND D.A4 GRIFFITH
NORTH M1ERICAN ~~EATHER CONSULTANTS
1141 EAST 3900 SOUTH, SUITE A130
SALT LAKE CITY, UTAH 84117
AND
JOHN A. FLUECK
FLUECK ASSOCIATES
WYNCOTE, PENNSYLVANIA
. AND
JACK A. HANNAFORD
SIERRA HYDROTECH
PLACERVILLE, CALIFORNIA
SEPTEMBER 1981
WS-2S0 (2.72)
Bure~u of Recl.&m&tloZl
1. REPORT NO.
4. TlTl.E ANO SUBTITl.E
S. REPORT OATE
North American 1~eather Consultants
1141 East 3900 South, Suite A-130 11. CONTRACT OR GRANT NO.
Salt Lake City, Utah 84117 7-07-83-V0008
I-;;r:-SP'Qi:;iSCiiRiNC-;:GiN;:::yi:iA-.:ii;--r;:;r;-r;:;;:;:;;;:='«----------~ 13. T Y P E 0 F REP0 RTAN0 PERI 0 0
112. SPONSORING AGENCY NAME ANO AOORESS COVEREO
INrERD1 REPORT ~K). 4
CONTINUING DESIGN WORK FOR THE SIERRA
COOPERATIVE PIWf PROJECT
7. AUTHOR!S)
R. D. Elliott, D. A. Griffith, J. A. Flueck,
and J. F. Harmaford
5. PERFORMING ORGANIZATION NAME 1.1010 AOORESS
Office of Atmospheric Resources Research
Bureau of Reclamation
ro Box 25007
Denver Colorado 80225
15. SUPPLEMENTARY NOTES
Septer.1ber 1981
6. PERFORMING ORGANIZATION COOE
8. PERFORMING ORGANIZATION
REPORT NO.
10. WORK UNIT NO.
Interim
a. SPONSORING AGENCY COOE
16. ABSTRACT
Research activities ot North Amprican Wpather Consultants (~AWC) and two sub~untrac­tors,
flueck Associates and Sierra Hydrotech, during the period October lY~O through
September 1981 are described. This research was concerned with continuing design
work on the Sierra Cooperative Pilot Project (SCPP) being sponsorpd oy the Burpau
of Hpclamation in the northern Sierra Nevada of California and ~evada.
Seeding scenarios are presented for aerial curtain seeding of major precipitation
bands and tor ground based seeding of post-frontal convective cells. It has bpen
determined that major bands commonly occur in the SCPP target area. The seeding
scenarios dpal with recommended treatment modes as well as estimates of potential
cloud seeding effects.
Two reports are presented which summarize work performed by NAwC and its subcontractors
as part of the detailed analysis performed on the SCPP during the 19~O-81 time
frame. These reports are concerned with dispersion of sepding material in the
SCPP and statistical design considerations of an exploratory phase of the SCPP.
NAwC and its SUbcontractors prepared five ditferent documents dealing with various
aspects ot the design of an exploratory phase on the SCPP. These documents discuss
the following topics: 1) treatment design, 2) a seeding guidance computerized
model, 3) seeding 'suspension criteria, 4) extra area effects of seeding, and 5)
statistical evaluations.
Two analyses are presented that were completed during the year dealing with estimation
of the required precipitation gage density and locations and the relationships
between various air mass characteristics versus time before or after upper-level
trough or frontal passage.
17. KEY WOROS ANO OOCUMENT ANAL.YSIS
a. OESeR IPTORS--
1~eather modification
Cloud seeding
Artificial nucleation
b. IDENTIFIERS-- /Project Skywater/Sierra. Cooperative. Pilot Project/Sierra
Nevada
c. COSATI Field/Group
18. OISi~l'i!l,\)~'t)~STATEMENT
Avoilable from the National Technical Information Service. Operations
Division, Sprin,fjel6, Virrinio 221SI.
NA
15. SECURITY C LASS Ill. NO, OF PAGE~
(THIS RCPORT)
U-N'CLASS IF lED
20. SECuRITY Cl.ASS 22. PRICE
(THIS 'Alitl
UNC LASSIF lED
TABLE OF CONTENTS
Section
ABSTRACT •••••••••••••••••••••••••••••••••••••••• i
1. INTRODUCTION .............•...........••......•.. 1-1
2. SCENARIOS FOR CURTAIN SEEDED MAJOR BANDS AND
G~OUND SEEDED CELLS •••••.••••.•••••••.••.•••..•• 2-1
2.1 Background and Conceptual Models ••.•.....•. 2-1
Hypotheses and Response Variates 2-43
2.3 Seeding Modes and Procedures ••••••••••....• 2-48
2.4 Estimate of Natural and Augmented
Precipitation and Frequency of Occurence ..• 2-60
2.5 Gaps in Knowledge ••••.••••••••••••••.•••••• 2-03
3.1 Task Force·6 - Transport and Diffusion •••••
3.2 Task Force 9 - Comparative Experimentation:
Some Principles and Prescriptions •••..••
3 • TASK FORCES 6 AND 9 ............................. 3-1
3-2
3-30
4. SCPP-1 DRAFT DESIGN APPENDICES .•..••.•••..••.... 4-1
4.1 Appendix D: Treatment Design ••.•••......•• 4-1
4.2 Appendix H: The Seeding Guidance Model ••.• 4-13
4.3 Appendix I: Suspension Criteria .•.•.•..•.• 4-35
4.4 Appendix J: Extra Area Effects ..••.••..•.. 4-58
4.5 Appendix N: Statistical Evaluation •••...•• 4-87
5. MISCELLANEOUS ANALySES.......................... 5-1
5.1 The Designing of a Precipitation Gage
Network for the SCPP •••••.••••••.••.••••.•• 5-1
5.2 SCPP and CRBPP Rawinsonde-Derived Parameters
versus Trough or Frontal Passages •••.•••.•• 5-13
REFERENCES •..••.••••.••••..•••...•••••..•.•....•• 5-17
i
List of Figures
2.1-1
2.1-2
2.1-3
2.1-4
2.1-5
2.1-6
2.1-7
Typical LWC and ICC distribution in
orographic convection ••.•..•.•••••• , ••,••••
Schematic of convection bubble model ••••. ••••••
Schematic of curtain being entrained
into bubbles , .
Seeding effects in parameter space •••••• , ••.•••
Distribution of particle mass ••••.•••••• , ••••••
TB-1 AgI curtain-orographic, curtain 30 krrl
downwind and 300 mb above valley floor.
Sounding type area wide-orographic .
TB-1 AgI curtain-orographic, curtain 30 kIn
downwind and 350 mb above valley floor.
Sounding type area wide-orographic •.••••••
2-8
2-10
2-11
2-13
2-18
2-20
2-21
2.1-8
2.1-9
2.1-10
2.1-11
2.1-12
2.1-13
2.1-14
2.1-15
2.1-16
TB-1 AgI curtain-orographic, curtain 30 kill
downwind and 400 mb above valley floor.
Sounding type area wide-orographic •••••••• 2-22
TB-1 AgI curtain-orographic, curtain 30 kn
downwind and 450 mb above valley flo~r.
Sounding type area wide-orographic ••.••••• 2-23
TB-1 AgI curtain-orographic, curtain 30 km
downwind and 500 mb above valley floor.
Sounding type area wide-orographic .•••..•• 2-24
TB-1 AgI curtain-orographic, curtain 0 km
downwind and 400 mb above valley floor 2-25
Aerosystems AgI curtain-orographic, curtain
30 km downwind and 350 mb above valley floor
Sounding type area wide-orographic •.••••••. 2-26
NAWC AgI curtain-orographic, curtain 30 km
downwind and 350 mb above valley floor.
Sounding type area wide-orographic •.••••••• 2-27
Nuclei production curve versus
temperature, TB-1 •••••••••••••••••.•.•••••• 2-29
Nuclei production curve versus temperature
for TB-l and Aerosystems silver iodide-acetone
generator •••••.••••••••••...•.•.••• 2-30
Nuclei production curve versus temperature
for TB-l and NAWC's silver iodide - acetone
generator , 2-31
11
List of Figures
2.1-17
2.1-18
2.1-19
2.1-20
2.1-21
2.1-22
2.1-23
2.1-24
2.1-25
2.3-1
2.3-2
AgI curtain-convection, curtain 15 km
downwind and 300 mb above valley floor,
top of convective instability at 350 mb
above valley floor. Sounding type -
major band 2-33
AgI curtain-convection, curtain 15 km
downwind and 300 mb above valley floor,
top of convective instability at 400 mb
above valley floor. Sounding type -
major band 2-34
AgI curtain-convection, curtain 15 km
downwind and 300 mb above valley floor,
top of convective instability at 450 mb
above valley floor. Sounding type -
major band 2-35
AgI curtain-convection, curtain 15 km
downwind and 300 mb above valley floor,
top of convective instability at 500 mb
above valley floor. Sounding type -
major band 2-36
C02 curtain-convection, curtain 15 km
downwind and 300 mb above valley floor,
top of convective instability 400 mb
above valley floor. Sounding type -
major band •••••.••••••••••••••••••.•...•••• 2-37
AgI point source-convection, ground source
20 km downwind and 40 mb above valley floor,
top of convective instability 350 mb above
valley floor. Sounding type - Ct ••••...•••• 2-38
AgI point source-convection, ground source
20 km downwind and 40 mb above valley floor,
top of convective instability 400 mb above
valley floor. Sounding type - C1 ••••••....• 2-39
AgI point source-convection, ground source
20 km downwind and 40 mb above valley floor,
top of convective instability 450 m above
valley floor. Sounding type - C1 ••••.•••••• 2-40
AgI point source-convection, xy plot.
Sounding type C1 •••••.••••••••••.••••.••.•• 2-42
Convective band schema tic •.••.•••••••••••.••... 2-49
Mean sounding for the 1980 forecaster
designated major band echo (PET) •••••••.•.• 2-51
iii
List of Figures
2.3-3
2.3-4
2.3-5
2.3-6
2.3-7
Possible seeding pattern in a major band •.•••.• 2-54
Mean vertical profiles of equivalent
potential temperature (solid lines)
displayed with low level maximum 9 e
isotherms (dashed lines) by forecaster
designated PET in 1980 •••.••••••••.••••••. 2-57
Mean sounding for the 1980 forecaster
designated cellular echo type (C1) •••..••• 2-58
Mean sounding for the 1980 forecaster
designated cellular echo type (C2) •••..•.. 2-59
Possible remote controlled ground generator
network 2-61
3.1-1
3.1-2
3.1-3
3.1-4
3.1-5
3.1-6
3.1-7
3.1-8
3.1-9
3.1-10
3.1-11
3.1-12
Orographic cloud concepts •.•.•••••••.••.••.••••
Precipitation change in fallout columns
with seeding (mm hr- 1 ) ••.•..•.......••..••
Typical stable flow pattern ••.•••.•••••••••••••
Typical V component ••••••••••.•••••••••••••••..
Typical flare expansion (circles) and
center dispersion (arrows) ••••••..••••••..
Typical curtain spread .••••••••.••.••••.••••.•.
Schematic of signature and fallout steps ••••...
Signature(s), 5 fallout tracks (FT), and
one fallout plume (FP), lower half of
curtain, orographic cloud •.•.•••.........•
Signature(s), and 5 fallout tracks (FT),
upper half of curtain, orographic cloud •.•
Horizontal section of signature(s),
precipitation track on ground (GT), 3 fallout
tracks (FT), and footprints upper and lower
half midpoints (FP) ••••••••••••.••..••.•..
Mushroom cell concept •• ~ •••.•.•••••••••. .•.•...
Precipitation (mm hr- 1 ) vs 2 D-C concentration
and liquid water content for different
3-3
3-4
3-5
3-6
3-8
3-9
3-12
3-15
3-16
3-18
3-19
3.1-13
3.1-14
c lauds 3-23
Signature and fallout tracks, lower half
of curtain, cell cloud ••••••••••••..•••••. 3-27
Signature and fallout tracks, upper half
of curtain, cell cloud 3-28
iv
List of Figures
3-37
3.1-15
3.2-1
Curtain dynamic effect .•••.••.•••••••••.••..... 3-29
The flow diagram of the contrast between
the "Investigator" and the "Real Word" ••••
4.1-1
4.2-1
4.2-2
4.2-3
4.2-4
4.2-5
4.2-6
4.2-7
Map of the proposed SCPP-1 target area with
a 10 km precipitation gage spacing. Gages
adj usted to 1-80 and US 50 ••••••.•••.•••••
Airflow normal to barrier ••••••.••••••••••••.••
Model predicted trajectories ••••.•••••.....•..•
Horizontal section •............................
Cell cloud and curtain at 2 times ••••.•••••••••
Typical cell history ••••.•••••••••••....••••.•.
Plot of cell runs .
Profiles of liquid water and ice along the
barrier (by step) for the case with no
4-2
4-15
4-15
4-16
4-17
4-18
4-19
4-22
4.2-8
4.2-9
updraft 4-21
Profiles of natural precipitation along
the barrier for the case with no updraft
Profiles of liquid water and ice content
along the barrier (by step) for the case
with an updraft •.•..••••••••••••.....•..•. 4-23
4.2-10
4.2-11
4.2-12
4.2-13
4.2-14
4.2-15
4.3-1
Profiles of natural precipitation along
the barrier for the case with an updraft ••
Seeded precipitation profiles for seeding
the case with no updraft with a C02 curtain
Seeded precipitation profiles for seeding
with C02 curtain, with an updraft •••••••••
Warm cell (with updraft) seeded precip-itation
for C02 curtain seeding at two
locations, and natural precipitation .•.•.•
Moderate cell (with updraft) precipitation
for C02 curtain seeding at two locations,
and natural precipitation •••••••••••••.•.•
Matrices of log of particle concentration
(NI m- 3 ) and log of particle mass (MI g) by
step (row) and by set (column) for a warm
natural cell case having no updraft ••....•
SCPP .experimental area ••••.••••••••••••.•..••.•
v
4-23
4-25
4-26
4-27
4-28
4-30
4-37
List of Figures
2.3-3
2.3-4
2.3-5
2.3-6
2.3-7
Possible seeding pattern in a major band ...•••• 2-54
Mean vertical profiles of equivalent
potential temperature (solid lines)
displayed with low level maximum e
isotherms (dashed lines) by forecaster
designated PET in 1980 •••••.•.••....•.••.• 2-57
Mean sounding for the 1980 forecaster
designated cellular echo type (C1) •••••..• 2-58
Mean sounding for the 1980 forecaster
designated cellular echo type (C2) •.••.••• 2-59
Possible remote controlled ground generator
network 2-61
3.1-1
3.1-2
3.1-3
3.1-4
3.1-5
3.1-6
3.1-7
3.1-8
3.1-9
3.1-10
3.1-11
3.1-12
Orographic cloud concepts •••..••.•.••••..••..••
Precipitation change in fallout columns
with seeding (mm hr- 1 ) ••.•.••••......••••.
Typical stable flow pattern •.•••••.•.•••..•.•.•
Typical V component •.•••.••••••••••••..••..•••.
Typical flare expansion (circles) and
center dispersion (arrows) •••••••.•..•••.•
Typical curtain spread •...••.....••..••.•••••••
Schematic of signature and fallout steps .•••.••
Signature(s), 5 fallout tracks (FT), and
one fallout plume (FP), lower half of
curtain, orographic cloud •..•••..••...••.•
Signature(s), and 5 fallout tracks (FT),
upper half of curtain, orographic cloud •••
Horizontal section of signature(s),
precipitation track on ground (GT), 3 fallout
tracks (FT), and footprints upper and lower
half midpoints (FP) •••••••...••...•••.•••.
Mushroom cell concept ••••.•••.•...••.•••.•••.•.
Precipitation (mm hr- 1 ) vs 2 D-C concentration
and liquid water content for different
3-3
3-4
3-5
3-6
3-8
3-9
3-12
3-15
3-16
3-18
3-19
3.1-13
3.1-14
clouds 3-23
Signature and fallout tracks, lower half
of curtain, cell cloud •••••••••••••....••• 3-27
Signature and fallout tracks, upper half
of curtain, cell cloud 3-28
vi
List of Figures
4.3-2
4.3-3
Historic flood conditions. Smith Fork
American River near Lotus, December, 1955. 4-41
Historic flood conditions. South Fork
American River near Lotus, January -
February, 1963 ..•......................... 4-42
4.3-4
4.4-1
4.4-2
4.4-3
4.4-4
4.4-5
4.4-6
Historic flood conditions. South Fork
American River near Lotus, December, 1964 •
SCPP primary experimental area ••.••.••••••...••
Sierra Cooperative Pilot Project primary
study area plus extended areas 1 and 2 ••.•
NWS precipitation gage network ••.••.•.••••.•.••
197~-80 SCpp precipitation gage network ••••...•
197~-80 SCPP meteorologicval network •.•...•..••
1979-80 cooperator's precipitation gage
4-43
4-64
4-65
4-66
4-68
4-70
4.4-7
4.4-8
4.4-9
4.5-1
4.5-2
5.2-1
5.2-2
5.2-3
5.2-4
network 4-71
Stations reporting hourly weather ••••.•..•••.•• 4-74
Rawinsonde sites ..........................•.... 4-75
Ground generators for SMUD, PG&E, and DRI ••
Schematic of the precipitation chain ••.•..••.•. 4-93
Probable location of the 95% confidence
region for two response variables •....•••. 4-95
Ice cloud, sounding cloud top, and water
cloud top for the SCPP 1976-80 versus
700 mb trough passage time ••••.•.....•••.• 5-15
Ice cloud top, sounding cloud top, and
water cloud top for the CRBPP versus
frontal passage time •.•••.•••.•....••.•••. 5-15
Combined SCPP PETS percent frequency versus
700 mb trough passagetimes ••••••.••.•..•.. 5-16
Mean seeded and not seeded precipitation on
the CaBPP for three groupings of precip-itation
gages versus frontal passage times 5-16
vii
List of Tables
Estimated total sample size for treatment
effects on a proportion (P) wi th a = .05
and S = .20 4-12
The performance matrix for the states and
components of a proper comparative
experiment 3-42
Primary response variables ••••••••••••.•.•••••• 4-11
2.1-1
2.1-2
2.1-3
2.1-4
2.1-5
2.1-6
2.2-1
2.2-2
2.2-3
3.2-1
4.1-1
4.1-2
4.1-3
Natural cloud top nucleation •••.••••.•••.•••.••
Orographic steady state water balance
( stable) .
Natural cloud top nucleation ••••••••••.••..••••
Orographic steady state water balance
(convective) .
Stable orographic summary •••••••••••••.•••••.••
Summary of unstable grid runs ••••••••••.••.••••
Links in seeding chain ••••••••••••.•••••.••••••
Major bands, modified curtain seeding ••••••...•
Cells, ground generator seeding ••••....•....•••
Estimated total sample sizes for ground-level
precipitation as a function of
treatment effect and power. The data. are
from 5 hour periods under C1/C2/ NE
conditions, 1978-80. The a level is
Page
2-3
2-4
2-6
2-7
2-28
2-41
2-44
2-46
2-47
• 0 5 and R = 0 ••• •••• ;..................... 4-12
4.3-1
4.4-1
4.4-2
4.4-3
4.5-1
4.5-2
4.5-3
Recommended cutoff for snowpack accumulatjon
at 180% of average for given date eXIlressed
as percentage of average April 1 watE~r
content for the American River Basin ••.•..
Physical - chemical evidence of Extra
Area Effects •...•.••••.•.......•••. I ••••••
NWS precipi tation gage densi ty ••••••••.• , •••.•.
NWS, SCPP and cooperator gages anticipated
to be in place for the 1981-82 season •.••.
Primary response variables •.••...•..•..• ,' .....•
Secondary response variables •.•.•.•••.•..' •..•••
Parameters to be estimated in SCPP-1 and
their suggested relation ••••••••••. , •.••.•
viii
4-49
4-60
4-67
4-69
4-89
4-90
4-91
List of Tables
5.1-1
5.1-2
5.1-3
General inputs to a network design ••..•....••••
Some empirical interception for various
precipitation gage network grid con­figurations
and spacings •••••..•••••.•..••
The frequency distributions of gage inter­ceptions
for various square grid spacing
Page
5-3
5-9
5.1-4
designs 5-11
The elapsed time of the cells in the
target area 5-12
APPENDICES
Appendix A -
Appendix B -
Appendix C -
Appendix D -
Some Theory of Grid Spacing
Network Grid Designs
The Precipitatin Data for Five Days of SCPP
in 1978-79 and 1979-80 Seasons
The Frequency Tables and Summary Statistics
for the Precipitation Print Variables
ix
Partial List of Abbreviations and Acronyms
AgI
AI
Bureau -
CRBPP -
DRI
ETI
ICC
IWC
LWC
MBA
NAWC -
NE
NWS
PET
PG1E ­SCPP
­SCPP-
l -
SMUD ­UW
silver iodide, a common weather modification
seeding material
Atmospheric Inc., Fresno, California
Bureau of Reclamation
Convective precipitation echo type (PET) with
less than 50% radar coverage over the American
River Basin
same as above except coverage greater than
50%
dry ice, a common wea ther modifica t:_on seeding
material
Colorado River Basin Pilot Project, a Bureau
weather modification research projec·c conducted
in the San Juan Mountains of Colorado
Deseret Research Institute, Reno, Nevada
Electronic Techniques, Inc., Ft. Colli~s, Colorado
and Auburn, California
ice crystal content
ice water content
liquid water content
MB Associates, Palo Alto, California
North American Weather Consultants, Salt Lake
City, Utah and Santa Barbara, California
no echo precipitation echo type (PET)
National Weather Service
precipitation echo type, a cloud classification
scheme developed for the SCPP
Pacific Gas and Electric Company
Sierra Cooperative Pilot Project
a planned first phase of an exploratory research
component of the SCPP
Sacramento Municipal Utility Distric;
University of Wyoming
x
1. INTRODUCTION
This report covers the activities of North American Weather
Consultants (NAWC) and its two subcontractors - Flueck Associates
and Sierra Hydrotech during FY 1981. The FY 1981 activities
on the Sierra Cooperative Pilot Project (SCPP) were different
in a number of regards from several of the previous years
in which the SCPP has been active.
Perhaps the most significant change resulted from a decision
by the Bureau that specified that FY 1981 would be an analysis
year instead of an active field data collection year. The
reason for this was the large amount of field data that had
already been acquired in prior years that had not been analyzed
in detail. This decision coupled with the desire to test
the feasibility of conducting one phase of an exploratory
program on the SCPP set in motion several activities.
The first action that was taken, based upon the decision
to perform analysis during FY 1981, was the organization of
nine separate task forces to address specific questions in
the analysis work. SCPP scientists, both Bureau as well as
contractor employees, were assigned to one or more of these
task forces. A lead scientist was appointed for each task
force. In NAWC's work Robert Elliott and John Flueck were
,
selected lead scientists on task forces 6 and 9, respectively.
Results of the work by the various task forces was reported
on at an analysis conference in Denver May 19-20, 1981. Presenta­tions
by Robert Elliott and John Flueck are contained in Section
3 of this report.
During the performance of the data analysis and out of
a need for adequate lead time to design and set up an exploratory
1-1
program for FY 1982, if such a decision to proceed was reached
in FY 1981, a parallel effort was initiated to construct a
number of possible seeding scenarios. These scenarios were
compiled by various Bureau and contractor personnel and presented
at two different meetings - preliminary scenarios at a special
meeting held in Salt Lake City on January 14-15 I 1981 and
at a design and task force workshop meeting conducted in Auburn
on February 10-12, 1981. Seeding scenarios were developed
for several different PETS (Precipitation Echo Types). NAWC
(Robert Elliott and Don Griffith) prepared one scenario concerned
with seeding major bands with aircraft and convection with
ground based sources. This scenario is contained in Section 2.
Shortly following the formal presentations of the task
force groups at the May conference in Denver, a tentative
decision was made to proceed with the design ot one phase
of an exploratory program for the SCPP starting in o:he 1981-82
winter season, designated SCPP-1. Consequently, work began
on the preparation of a draft design for SCPP-1. Various
Bureau and contractor employees then were requested to prepare
design appendices on a variety of sUbjects to pro'Tide backup
for a design prepared by Larry Vardiman of the Bureau. NAWC
and its subcontractors participated in the preparation of
five such appendices as follows: Appendix D - Trea:ment Design;
Flueck Associates - John Flueck; Appendix H - The Seeding
Guidance Model, NAWC - H.obert Elliott; Appendix I .. Suspension
Criteria, NAWC - Don Griffith and Sierra Hydrotech - Jack
Hannaford; Appendix J- Extra Area Effects, NAWC - ~n Griffith;
and Appendix N - Statistical Evaluation - Flueck Associates ­John
Flueck. These appendices were discussed at a draft design
workshop held in Auburn on July 14-15, 1981. The fi VI:! appendices
are presented in Section 4.
1-2
Some other analyses and developmental work was performed
during the reporting period. Notable among these was the
considerable revision to Robert Elliott's seeding guidance
model. Output and discussion of the model appear in Sections 2,
3.1 and 4.2. John Flueck of Flueck Associates also conducted
a study of the possible desired precipitation gage network
needed to detect precipitation from postfrontal convective
cells (C1 and C2 PETS) during SCPP-1. This work is summarized
in Section 5 and Appendices A-D.
1-3
2. SCENARIOS FOR CURTAIN SEEDED MAJOR BANDS AND GROUND SEEDED
CELLS
2.1 Background and Conceptual Models
The data available for establishing scenarios for full
scale exploratory experiments con~ist primarily of the Sheridan
soundings, air and ground microphysics, radar, and surface
network data. At present only advance and partial analyses
are available so that whatever is designed at this point would
undergo some, but not necessarily radical, change before possible
implementation in the coming winter season.
Observations are for the most part made on a regular
schedule during storms and cover a large portion of the experi­mental
area. However, the cloud microphysics by necessity
is confined to a very small volume swept out by a high speed
aerial platform. The most extensive sample of data for convective
systems is that for cells, while the most limited sample is
that for the major band, the most complex system. The latter
were sampled primarily during January and February 1980, a
period of time when storms came in a series of jet stream
(EJ) weather type, notable for the very tropical maritime
character of its air mass.
In what follows the results of various analyses, mostly
preliminary in nature, will be invoked without giving any
details since the details have already been presented at various
meetings.
Because the seeding process commences by altering cloud
microphysics, it is obligatory first to consider natural micro­physics
for the convective case.
2-1
2.1.1 The Orographic Scale. Scatter plots of super-cooled
liquid water content (LWC) versus ice crystal concentration
(ICC) indicate supercooled liquid water to be most prevalent
in the 0 to _5°C zone, next most prevalent in the _5° to -10°C
zone, and least above. An analysis of the top of the water
saturated cloud as given by the Sheridan sounding shows a
similar frequency distribution. It can be expected that water
saturated cloud would be associated with liquid cloud droplets.
In addition, comparisons of near simultaneous Sheridan and
Freshpond soundings show that the water saturated cloud thickens
on ascent. This observation, along with observations of frequent
riming at ground level high on the barrier, suggest considerable
supercooled liquid water accumulation, in general, at levels
lower than those sampled by the aircraft.
The accumulation of LWC at lower levels up the barrier
is simulated quite well by means of a simple orographic wa ter
balance model (OROGWATER). Some calculations will be discussed.
Table 2.1-1 shows the steady state distribution of LWC
calculated for a cloud with a top 5000 m above va.lley floor
and typical orographic sounding parameters. Na"tural cloud
top nucleation occurs at the top of each of the sev{~n "fallout"
columns, which are literally slanted to the right as one descends
the column. LWC does accumulate at low levels as Ole proceeds
up the barrier, reaching a peak of .5 gm- 3 near the crest.
A simple simulation of seeding is to multiply the natural
nucleation by a factor greater than 1. When this is done
the new steady state distribution of LWC and the new ground
level precipitation appears. Table 2.1-2 shows the results
of several different seeding schedules. The first schedule
multiplies natural nucleation by 5 for 4 columns (each 13 km
2-2
Table 2.1-1. Natural cloud top nucleation.
Distribution of LWC (gm-3)
0.010 0.002 0.003 0.013 0.031 0.052 O.OTI
0.020 0.007 0.007 0.019 0.045 j). Q.S2_ / /6':'141
0.030 0.007 0.020 0.038 0.070 / 0.109 0.179
0.040 0.017 0.023 0.054 .Q .(L~3 ..... ..- 0.132 0.199
0.050 0.019 0.028 0.065 , ... 0.104 0.138 0.194
0.040 0.022 0.057 0.021 ..- 0.102 0.128 0.175
0.030 0.015 0.068' 0.034 I 0.180 (0:271 / /" I" O. i33
0.020 0.010 -0.-050- __0..D'Z..4 .... o 158 0.090 0.128 0.010 0.005 <-.Q·10.2 .218 .' / 0.350 / : 0.433 ...0.511
0.000 0.000 0.086 ........ _ .127 0.141 0.079 . /0.392
Distribution of Nuclei
2 1 1 1 1 ..,
w
1 1 1 1 1 1
1 1 1 1 1 1
1 1 1 1 1 1
1 1 1 1 1 1
1 1 1 1 1 1
1 1 1 1 1 1
1 1 1 1 1 1
1 1 1 1 1 1
1 1 1 1 1 1
2-3
r-SEVEN FALLOUT CHANNELS ~
CASE
NUMBERS
CREST
TEN FLOW
CHANNELS
I 2 3 4 5 6 7 ITEM
I I I 5 5 5 5 SEED !;CHEDULE
.18 1.35 2.87 3.25 3.90 3.84 - NATU~iAL (mmh-I)
0 0 0 -.02 +.54 +.41 - CHAN(iE IN F.O.C.
0 0 0 0 -.02 +.54 +.41 SHIFi
I I I 10 10 10 10 SEED SCHEDULE
0 0 0 +.01 -.12 +.66 - CHAN(;E IN F.O.C.
0 0 0 0 +.01 -.12 +.66 SHIFl
I I I 100 .100 100 100 SEED SCHEDULE
0 0 0 +.33 -.57 +.79 - CHAN \;E IN F.O.C.
0 0 0 0 0 +.33 -.57 SHIF1'
TYPE--STABLE
FLOW TOP(m )-5000
CLOUD TOP (m)---5000
DEAD LYR. (m) . 500
Table 2.1-2 Orographic steady state water balance (stable).
2-4
wide) over the region where LWC has accumulated in the natural
case. Some enhancement of precipitation rates is indicated.
But the fallout trajectories are also lengthened downwind
because of the smaller ice particle sizes, and this shift
is crudely simulated in the 4th row of this schedule. The
7th column (not computed) would actually be shifted to the
lee size, enhancing precipitation there, but reducing the
upwind net.
The next two seeding schedules are for the same pattern
but 10-fold and 100-fold factors. The pattern of precipi­tation
change is erratic (due probably to the finite size
boxes employed) and difficult to interpret. There is a suggestion
that a five-fold increase is optimum.
The change in the water balance resulting from embedded
convection can be simulated by inserting in OROGWATER a mean
low level foothill convective updraft of 1/2 ms- 1 • Table 2.1-3
shows how the LWC accumulation now reaches a maximum in the
convective region. The lower portion of the figure shows
an interesting ICC wake that was simulated by permitting a
10-fold ice multiplication whenever the particle mass reaches
10- 5 g between the -40 and _7 0 level. This conforms to one
of the rlallett-Mossop process conditions.
Table 2.1-4 shows the results of applying different seeding
schedules. The first two are over the foothills, and the
third downwind. The foothill case shows the most promise.
A point of interpretation of the OROGWATER model output
is that the fallout channels pertain only to the larger particle
sizes, such as are best represented by the observed 2D-P concentra­tions.
The 2D-C particles would float laterally across the
2-5
Table 2.1-3. Natural cloud top nucleation.
Distribution of LWC (gm-3)
0.010
0.020
0.030
0.040
o 5
0.040
0.030
0.020
0.010
0.000
0.002
0.007
0.007
0.017
0.013
0.019
0.038
0.054
0.031
0.045
0.070
0.093
0.052
0.082
0.109
0.132
0.079
0.141
0.179
0.199
Distribution of Nuclei
2 1 1 2
1 1 1 1
1 1 1 1
1 1 1 1
1 1 1 1
1 1 1
6 5 1
6 5 1
5 5
5 4 5
2-6
r---- SEVEN FALLOUT CHANNELS ~ CREST
CASE
NUMBERS
TEN FLOW
CHANNELS
I 2 3 4 5 6 7 ITEM
5 5 5 I I I 1 SEED SCHEDULE
3.27 5.48 3.51 3.06 3.12 4.88 - NATURAL (mmh-I)
-.16 +.21 +.16 +1.0 +.52 +.83 - CHANGE IN F.O.C.
-.16 +.21 +.16 +1.0 +.52 +.83 SHIFT
10 10 10 I I I I SEED SCHEDULE
+.15 -.47 +.52 +.27 -.32 -.98 - CHANGE IN F.O.C.
+.15 -.47 +.52 +.27 -.32 SHIFT
I I I 5 5 5 I SEED SCHEDULE
0 0 0 +.07 +.02 +.22 - CHANGE IN F.O~C.
0 0 0 0 +.07 +.02 +.22 SHIFT
TYPE--CONV~
FLOW TOP{m )-5000
.. WITH I.M. AT 10"5g BETWEEN -40 AND -70 •
CLOUD TOP (m)---5000
DEAD LYR. (m) 500
Table 2.1-4 Orographic steady state water balance (convective).
2-7
fallout columns. A seeding signature would represent the
latter situation,as would many small ice crystalB at cloud
top. The question of whether a stochastic grow-:h process
is active in the conversion of 2D-C size particles in the
signa ture to 2D-P fallout particles must be addreSSE!d. Ground
level microphysics fits the conversion concept quite well.
Initial simulations permitting a 10% conversion in fi.ve minutes
leads to resul ts fi tting observed ground level mil~rophysics
and signature behavior.
The aerial microphysical data show that in nany cases
there exists a layer of abundant ice crystals between the
_5°C and -10°C level. When individual cases are reviewed
there almost always appears to be evidence for convection.
sometimes quite strong, but at other times relatively weak,
upwind of the crystals. This phenomenon is not to be confused
with thick iced-up stable orographic or area wide clouds which
may be converted to ice due to cloud top nucleation. The
phenomenon is depicted in Figure 2.1-1.
Fig. 2.1-1 Typical LWC and ICC distribution in orogra~hic convection.
2-8
Usually, the convection lies over the foothills where
supercooled liquid water is concentrated, while there is a
wake zone of high ice concentration stretching up along streamlines
almost to the crest and sometimes even beyond. The base appears
to be at the -50 level near convection but rises along stream­lines.
The LWC in the convection is variable but ranges from
a few tenths gm- 3 to sometimes over 1 gm- 3 • Updrafts range
from a half ms- 1 to 4 ms- 1 or more. The height at which conversion
to ice occurs varies apparently with air mass type, but also
must depend upon how the sampling is done. In the icy wake
2D-C concentrations range from 20 1- 1 to sometimes several
hundred. The 2D-P concentration runs about 10% of the 2D-C.
The Convection Scale. In order to simulate
seeding effects in single cells, nucleation and microphysical
changes in a rising convection bubble have been calculated
in the manner depicted in Figure 2.1-2 (BUBBLE model). It
is assumed that a 500 m deep convection bubble is produced
in the inflow region at convection base which rises in a series
of steps, starting and ending at positions spaced 500 meters
apart. At the top there is an outflow region where the smaller
ice particles produced in the updraft (IWC) and the remaining
liquid cloud droplets (LWC) pass into the dry environment
and evaporate. The larger precipitation size particles (>
10- 5g) fall to the ground out of the outflow region (PWC).
In the case of emergent convection they fall a considerable
distance in dry air, but are too large to completely evaporate
before reaching any lower orographic cloud, where they may
grow further by accretion.
In this simulation new particle sets (naturally or arti­ficially
nucleated) are produced at each step, in accordance
with the step mean temperature. In the natural case the standard
2-9
HEIGHT IE ;) DISTANCE
POSITION NO.
STEP NO.
4
3
2
-I
INFLOW
-3
-5
OUTFLOW
\PWC
~ LViC
~IWC
Fig. 2.1-2 Schematic of convection bubble model.
2-10
exponential background formula is used, but ice multiplication
between the -40 and -80 C levels can also be introduced. With
artificial nucleation a curtain that has been in existence
long enough to produce a reasonable (and computable) average
ice nuclei concentration is introduced near the base of convec­tion.
A curtain of less thickness than the 1 km standard
used in the calibration runs would be preferable; i.e., its
depth should be comparable to the inflow region depth. The
curtain is assumed to have expanded to where its area is sufficient
to encompass one or more newly developing bubbles. This may
require an hour. Figure 2.1-3 depicts the process.
CURTAIN
Fig. 2.1-3 Schematic of curtain being entrained into bubbles.
2-11
The nucleation and growth of each set of paJ~ticles is
computed stepwise. There may be one set nucleated in step
one, another in step two, etc. If there are 10 steps to the
outflow region, then there will be 10 sets of particles there.
The particle spectrum is thereafter divided into the three
aforementioned categories and the water content of each calculated
(PWC, IWC and LWC). The crucial one is, of course, the precipi­tation
water content (PWC).
An important part of BUBBLE is the entrainment factor,
a factor of obvious importance in the Sierra where the total
water content (LWC and IWC) in convection is observed to be
much lower than the adiabatic value. The momentum of the.
environmental wind (2/3 cellular mean sounding) is entrained
and mixed at each step with that advected up from below so
that the horizontal motion of the bubble can be char":ed.
Computations were made to compare seeded and natural
production of PWC at the outflow region position (and at all
lower positions) for a variety of updraft rates and cloud
depths. The convection base was set at the 850 mb level.
Figure 2.1-4 shows the results wi th respect to area.:; favorable
or unfavorable for positive seeding effects on a cloud depth
versus updraft plot. Total bubble duration lines are also
shown. These lines slope because duration depends upon both
updraft and depth. The duration includes residence time in
an outflow region that extends outward to where the prl~cipitat ion
particles have fallen out of that region. This adds about
1000 sec to the whole duration. It is estimated fron presently
available SCPP climatology that the vertical scale embraces
about 80% of convection cases and the horizontal scale about
the same for updrafts.
2-12
3000 5000
POSTIVE Eo(-­EFF
CT
2000
3500 -19
4500 -24.5
4 1.6
UPDRAFT (ms-I)
1.0
Fig. 2.1-4 Seeding effects in parameter space.
2-13
Figure 2.1-4 indicates that in about half of the total
area positive effects are expected. Such ef fec ts are s,ubstan tial
(several fold), whereas in the rest of the area, seed-natural
differences are small and negative.
The effect of dynamically produced enhancement of buoyancy
can be visualized by reference to Figure 2.1-4. A rise in
the top appears to reduce seeding effectiveness (or E~nhancement
of precipitation efficiency), but since the overall condensation
(and hence PWC) increase s wi th depth, the net dynamic seeding
effect is posi t i ve. However, if there is a stable lE.yer alof t ,
as seems to be the more normal case in the Sierra, then there
results an enhancement of updraft leading to a wider dispersion
of the PWC in the outflow region, which effect is in keeping
with recent detailed 2-D numerical model results (Eirh-Yu
et a1, 1980).
In. t he case of bands the dynamic ef fec ts require more
complex mesoscale modeling procedures, and the working conceptual
model to be used herein is based upon them (Fritsh and Chappell,
1980; Cotton and Tripoli, 1980).
The entrainment factor used was roughly a dDubling of
the bubble area per two steps (1000 m) for a 1 kHl starting
radius. This is in line with general observations, but may
be low for Sierra storm convection. A 1 km starting radius
was used. The center of the outflow region was uS"lally 30-40
km downwind from the starting point, and 20 or so km downwind
of the lower level drift. Should there be a continuing updraft,
rather than a bUbble, then the precipitation particles falling
from the outflow region (which itself is centered 10 or more
km downwind of the higher steps) would fall wei] ahead of
that updraft. However, in low wind shear cases, g"raupel type
2-14
precipitation particles could fall into the updraft. If this
were the case, and a Hallett-Mossop type generation of 10
particles of 10-7g size for each graupel particle is permitted
in the -40 to -80 C zone, then the natural case would be seeded
by these additional particles. A calculation was made assuming
104 graupel particles per m3 were introduced, and all of the
previous runs recalculated. The seed-no seed differences
in PWC in the outflow zone were now very small, with some
negatives. The ice mUltiplication had the effect of overseeding
the long duration natural cases with little S-NS difference
resulting.
For intermediate durations the same was true, but for
short durations there was no overseeding effect; there was
substantial precipitation, and seed-no seed differences were
nil.
Overall, these calculations suggest that when this ice
multiplication system is fully effective, seeding will not
compete with it successfully. The key predictive factors
for this type of ice mUltiplication seems to be the wind shear,
the depth of convection and the air mass type. The latter
has not been precisely defined, however recent analyses of
the distribution of sounding-defined water saturated cloud
top suggests that it may occur most frequently when cloud
bases are low (high cloud base saturated mixing ratio). This
is in keeping with previous work relating cloud base mixing
ratio to the prevalence of large drops produced by the condens­ation-
coalescence process. Some preliminary analyses of water
saturated cloud top distributions indicate that there is a
potential for these ice multiplication effects when the cloud
base mixing ratio exceeded 6.5 g/kg, and according to the
sounding data this occurs nearly half of the time, although
2-15
,.,.
this may be climatologically slanted toward the tropical maritime
type air mass. However, unless fallback into the lpdraft is
assured, the result would be only to produce many small ice
cry s t a lsin the -40 to -8 o C range of any orogra:;>hic cloud
catching the fallout.
Tentatively, it appears that fallback into ~onvection
is assured for convection too shallow to extend m'~ch higher
than the _8°C level. This would be characteristic of embedded
band convection. However, for the deeper convection geometrical
factors preclude any fallback unless the vertical wind shear
from convection base to top is less than about 3 Ms- 1 • This
still leaves open the question as to the possibility of graupel
falling from the top or one cell reaching the lower levels
of another cell.
In bands there is a mesoscale circulation su.perimposed
upon the orographic flow, and containing individual eell circu­lations.
This would help to distribute fallout particles
to downwind cells. But these may be primarily d~'ing cells
lying in the mesoscale outflow region, and most likely are
already heavily iced.
In the C2 cellular case, there also appears to be a mesoscale
pattern, with a line of new starting cells lying in the foothills,
and older decaying cells drifting downwind up the slope. To
play an important role in the overall water balance, ice multipli­ca
tion would have to occur in the starting 'line.
Without fallback into convection, there would still be
the possibility of this type of ice multiplication within
any orographic cloud present. This would merely produce numerous
ice needles in the -40 to -80 C range, and could ~~~uce LWC
2-16
at lower levels. This development could account for the high
ice concentration wake that moves up streamlines as indicated
in Figure 2.1-1.
Considering all factors, it is presently estimated that
of the 50% or so cases where the air mass favors this type
of ice multiplication, 50 to 60% would result in graupel fallback
into active cells and resultant ineffectiveness of seeding.
Thus, cases favorable for seeding would be about one third
overall.
The delineation of areas of seeding response indicated
in Figure 2.1-4 and in the text (based upon cloud base mixing
ratio and wind shear values) are provided as elements of a
conceptual model. They can be used to form testable hypotheses.
It is conceivable that refined modifications of this model,
or use of different key inputs (e.g.; starting radius, entrainment
factor, cloud base, etc.), or entirely different models, can
supply somewhat different testable hypotheses within the same
key parameter framework. These should and can be developed
prior to commencement of an exploratory phase.
The model calculations provide a microphysical output
that can also be compared to observations obtained by aircraft.
Figure 2.1-5 is an example of the ice particle distribution
in the outflow zone for a single case. The solid line compares
well with the exponential type distributions observed. The
dashed curve is the ice water concentration per particle set,
i.e., the product of the paricle number concentration and
the mass. It is fairly level through a range, and this fits
observations in general. Curves such as these for various
parameter combinations, can serve as the basis for formulating
testable hypotheses. More importantly, the progressive change
2-17
-4 -3 -2 -I
LOG MASS (g )
-5
-
-
-
-
-
-
f---- ,
L. __,
L ___,
r----~ L.___,
~______...J
I
-I I I I I I
-6
4
2
LOG CONC
(NO M-3)
5
3
-3
-2 0
LOG CONC
X MASS
( gm-3 )
-I
Fig. 2.1-5 Distribution of particle mass.
2-18
in the distribution with time (or height of the bUbble) can
be predicted and tested. In the runs there was a bulge in
mass curve (dashed) that progressed to the right with time.
2.1.3 Combined Convection and Orographic Scale.
A further exposition of the conceptual basis for both band
and cell seeding is given by a series of seeding simulation
by the GUIDE model (see Section 4.2 for a description). Figures
2.1-6 to 2.1-13 show the results of seeding in a stable orographic
cloud with TB-1 flares in curtain mode from different heights
over the foothills, using different nuclei sources. The calcu­lations
are made only for curtain midpoint and to get a complete
effect per curtain several runs at different levels are needed.
Clearly for TB-1 the optimum fallout pattern over the barrier
occurs when the curtain is centered at 350 mb above the valley
floor. Figure 2.1-11 shows that moving the source back upwind
produces little change. Table 2.1-5 summarizes the figures.
Figures 2.1-12 and 2.1-13 pertain to different generator
types (which could be used to line seed, but not to produce
a curtain). Figures 2.1-14 through 2.1-16 show the production
curves (starred points) for TB-1, Aerosystems, and NAWC generators,
respectively. The TB-1 flares have, in general, the lowest
nuclei production per gram. However, the typical output in
grams per second is the greatest. The best comparison between
types is obtained by multiplying the production figures by
this output. The auxiliary curves on Figures 2.1-15 and 2.1-16
make this adjustment and show that the types are more nearly
equal in application than the vast differences that their
production curves would suggest (although unit costs to achieve
these production curves can vary substantially).
2-19
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4.01l£+0c? •
1.00E+02 •
~ 3.00l+0c?·
~
•.-i
~
~ c?OOE+02 •
I
~
~
distance (krn)
Fig. 2.1-8 TB-l AgI curtain-orographic, curtain 30 km downwind and
400 mb above valley floor. Sounding type area wide-orographic.
-------------------------------------------------------------------------------------------------------- I 1 1 I 1 1 1 I I
l.\j
I
l.\j
W
b.OO[+Ul • •
.,.00E+02 •
4.00[+02 •
,-."
..0
S
'--J
+oJ ).00E+02 •
...c:
bO
'rl
~
l.uOE+Ui> •
1.00[+0<' •
30,450 • • 2 .2 •
-240
2
~ . ••
.. . 2
2 • •
•• 2 •
.-
• • •
2 •• • 5·
/I 1
2.2. 2 2] 1
• 1
1
MISS
TB-l
O.OOE+OO -:_qI -I --l_:.- l • ~~ I ~~ I~ I 1 1 1 1_1
O.OOE+OO 2.QO£+01 4.80E+OI 7.20E+Ol Q.bOE+Ol 1.20E+02
distance Ckrn)
Fig. 2.1-9 TB-l AgI curtain-orographic, curtain 30 krn downwind and
450 mb above valley floor. Sounding type area wide-orographic.
----------------------------------------------------- --------------------------------------- I I I 1 1 I 1 I 1 I
f, .00[+02 •• " -
-270
5.• 0 OE +02 •
,,--..
~
'-.-/ 4•• 00E+02 •
30,500 • "
"""2
"
3 2
?
• 2
3
2
2
"
2 2
• " 2
2 q
(,I
~
bO
'M
Q) r.c: 3\ • 00 E+02 -
,-
MISS
TB-l
(').00[+00 •1_1~I"l_: I * ~_I I 1 ~ 1 1___ 1 ' 1_1
O.OOE+OO 2.40E+Ol 4.AOE+Ol 7.20£+01 Q.hOE+Ol 1.20E+02
. 2. OOE +02 -
1.00E+02 •
[:\J
I
[:\J
~
distance ekm)
Fig. 2.1-10 TB-l AgI curtain-orographic, curtain 30 km downwind and
500 mb above valley floor. Sounding type area wide-orograpllic.
----------------------.----------I--------- --------~---------I---------I---------.---------.---------~
6.00£+02 - •
5o.00E+02 -
•
-14.5°
. -
0,400
'<l. OOE +02 -
,-.,.
il '--'
~
..0..,0
~
~,00E+02 - . • 2
'. • • • 2 2 ••
3 2 2 • 2 • • ••
2 3 ••• •• •• i! ••
• •• •• • •
• • • •
• • •
i! ? •
.~ ••• • •
• •• • ••• • • •
• • • •• ••• • • •
• • • •• • ••• • •
• • 2 .2 •••
• • •••
2
1
• 1
• • 1
• 1
2 -
rv
I
rv
CJ1
f.00E+02 -
1.00E+02 -
MISS
TB-1
2·
0.00£+00 -1_1~1_-.--_--__ 1 -----1---- 1 --1 1 1 • 1 1 1_1
0.00£+00 2.110E+01 1l.P,OE+01 1.20E+OI '1.1>0£+01 1.201:.+(12
distance (km)
Fig. 2.1-11 TB-l AgI curtain-orographic, curtain 0 km downwind and
400 mb above valley floor. Sounding type area wide-orographic.
~I---------I---------~--------~--------I----------I---------I----------I--------- ----
6.00E+02 .. • • ·..
5.00E+02 -
'·2·
199 AF
AEROSYSTEr",
-12.2
• • • • •
• • • 2 •• •• • 2 • I
2 2 2 2 • •• • •• • • 2 2 • I
• • 2 • ~ • • • • • I
• • •• •• • • I
• • • • • .2 •• • ..
• • • 4 • • I
• • • ~ • / I
2 I
• ~ ..... I
30,350 •
.'2"
O.OOE+OO -I_I~I.I I__ , ~ ~_I I ----1 ---_--1_--- --1--- 1_ I_I
O.OOE'OO 2.QOE+01 4,60E'Ol 7,20E+Ol Q,bOE+Ol 1.20E+02
'1.00E+02
1.00E+02 ..
2.00E+02 -
r--.. g
+oJ
...c=
b1)
•..-f 3.00E+02-
t\:) ~
I
t\:)
O'l
distance (km)
Fig. 2.1-12 Aerosystems AgI curtain-orographic, curtain 30 km downwind
and 350 mb above valley floor. Sounding type area wide-orographic.
j-T---------T---------s---------T---------j---------T---------T---------.----------I---------.---------~
Ii. 0 I) [ + 0 2 • * . * • •
5.00[+02 -
r---
~ ~.OOE+02-
'-'
-12.2
- - - - - * - 2 ** ** 2
- 2 2 3 *2 * _. * -2
2 • .. • 2
• • _.*
* • • •
2 * • • IIII
2
*-
***2*
2 2
**
5.5 AF
**-
NA\\IC
•
30,350
~.OOE+Oi.' -
1.00E+02 •
;I.Ou[+02 -
~
Oll
0"';
]
I.\:)
I
I.\:)
-J
O.UO[+OO •l_l.,......-- I I - I I * ~ I 1 1 1 1_ I_I
O.OOE+OO 2.40E+01 4.80E+Ol 7.20E+Ol Q.60E+Ol 1.20E+02
distance (km)
Fig. 2.1-13 NA\VC AgI curtain-orographic, curtain 30 km downwind and
350 mb above valley floor. Sounding type area wide-orograp]lic.
Table 2.1- 5. Stable orographic summary of Guide nms.
Nucleating Coldest Precipitation
Figure Method X (km) Z (mb) Temperature °C (acre feet)
2.1-6 TB-1 30 300 - 8.6 7.9
2.1-7 TB-1 30 350 -12.2 130
2.1-8 TB-1 30 400 -14.9 0
2.1-9 TB-1 30 450 -24.0 0
2.1-10 TB-1 30 500 -27.0 0
2.1-11 TB-1 0 400 -14.5 0
2.1-12 Aero- 30 350 -12.2 199
systems
2.1-11 NAWC 30 350 -12.2 5.5
(ground
based
acetone)
2-28
-------------------------------
l.bOE+OI - •
------- ------------
* -
~
1;'0
l-<
Q)
p..
Vl
..--i
CIl
+J
Vl >-.
l-<
U
t\j Q)
I U
t\) H
(,0
1.50£+01 -
I.QO£+OI -. •
1.]0£+01 -
1.20£+01 -
1.10£+01 -
1.00£+01 - *
• * •
* * *
* *
* *
* * '*
*
*
*
*
1_1 I I l I l l * * * * * * - ~I 1___ I I_I
-2.50£+01 -2.00£+01 -1.50£+01 -1.00£+01 -5.00£+00 0.00£+00
Temperature CQe)
Fig. 2.1-14 Nuclei production curve vs temperature, TB-1.
------------------------------------------- 1 1 1 1 1 1 1 1
J .bOE+OI - *
._----.1--1
_ -2.16
•
~ AEROSYSTEMS
*~ (.035 Gs-l) •
TB-l
(10 Gs-l)
• • * • * • • •
-----
ADJUSTED AEROSYSTEMS /
•
-- ..... ~---
* *
--- I. ",OE+OI •
I.IIOE+OI •
I.I0E+01 • \
\
I.OOE+OI -. \ -
1_1 __--_--__ 1_--------1_-----_--1_-------_1 ----1_--------1 -----1_--------1--- I I_I
-2.50E+01 -2.00E+OI -1.50E+01 -1.OOE+Ol -5.00f.+00 O.OOE+OO
1
1 *
1
1
1.50E+01 -
1.20E+01 -
@
I-<
bll
I-<
Q)
P<
If)
M
cd
~
If)
>.
t\j
I-<
I
U
W
Q)
0
U
H
Temperature (OC)
Fig. 2.1-15 Nuclei production curve vs temperature for TB-l and Aerosystems
silver iodide-acetone generator.
--J-------i-------..,..l--~----..-.-J----I------J---I---
l.bOE+OI - 2 • • • • • • • •
._----- 1 1
* •
-3.35
• • • *-
NAWC
-1
(.0045 GS )
•
•
./
• •
TB-l
(10 GS- 1 )
•
ADJUSTED ~
NAWC ~,
"\
\
-- ~ - --- .-. .----. ... ~
I.IOE+OI -
I.OOE+OI1-_*1 1 1 1 1 1 \ 1 -:...1 ., •1 • 1 I_I
-i!.SOE+UI -2.tJOE+01 -I,50E+UI -I,OOE+OI -5.UOE+00 .0.00£+00
1.20E+01 •
1.30E+UI • --. •
1.50E+OI •
1.40E+OI -
~
1;'0
I-<
(])
p..
1Il
...-i
cd
~
1Il >- I-<
U
(])
U
l\:)
H
1
W
......
Temperature COC)
Fig. 2.1-16 Nuclei production curve·vs temperature for TB-l and NAWC's
silver iodide-acetone generator.
Figures 2.1-17 through 2.1-20 show the results of GUIDE
runs for the convective case. Various convective illstability
tops are covered for the TB-1 curtain case, in which the mean
major band sounding was employed as an i npu t • F i gllre 2. 1-21
pertains to a C02 curtain, but allows it to be ef1ective for
a longer time than can be expected. In all cases the curtain
midpoint is allowed to remain within convection (often at
its top) for 2000 sec, and then allowed to pass into the environ­mental
cloud. A mean updraft LWC of 0.5 gm- 3 and updraft
speed of 0.1 mbs- 1 was employed.
Figures 2.1-22 through 2.1-24 pertain to grounc generator
seeding (NAWC type) of convective cells. The mean C-2 (2/3
cell coverage) sounding was used as an input. Table 2.1-6
summarizes the figures.
Figure 2.1-25 plots the fallout on horizontal (X-,Y) section.
It is clear that the model can be useful in posit'Loning the
release so as to target a specific area. This brtngs up the
very important matter that the upward-downward motion cycle
of nucleant and ice particles is helical in form.
The above Guide model runs are not intended to be detailed
guides for the seeding scenario. They only illustrated key
aspects of the conceptual model. The Guide model would be
used during the course of an operation to supply detailed
guidance as to seeding mode and procedures in a give!l case.
Guide does not give any but meager guidance on how to
exploit the dynamic effects potential of the unst~ble case.
The most likely place for seeding to generate an enhancement
of buoyancy, and therefore a dynamic invigoration and expansion
of the system, is in the high LWC area in the foothills (or
2-32
----------------------------------------------------------------- -----------------~- I I , I I I I I I I
~.OOE+02 - " "
._----- 1"
-
" 2- 7*3245. 5? 2 .-. 2
.:3. 2 *2** 24 *" " 0* r? ".2" *
*2" * *
* 3*
" " ~ *
-8.80
CIT 350 - * * * .. * 2 2 .. * .. 2 2 5***" 2?
" " * .. .. * * * 3 *2 3* 322 *2 **2 2
* * * * ..
15,300 ,,* * * *
* * * *
* " *
* .. " "
* " ..
"
" *
~ *"34 5 2*2
O.OOE+OO •1_.1_.-----I--. -_-_I * I - I * ~ -_I_~ 1 1 1 1 1_1
O.OOE+OO 2.QOE+Ol 4.80E+Ol 7.20E+Ol Q.ftOE+Ol 1.20E+02
1.00E+02·
5.00E+02 •
I
I1I
r-'"'\ 1+.00E+02 -
~
'--'
~ ...c:
b.O '3. OOE +02 • 'M
~
t'V
IWW
2.00E+02
distance (km)
Fig. 2.1-17 AgI curtain-convection, curtain 15 km downwind and 300 mb above
valley floor, top of convective instability at 350 mb above valley floor.
Sounding type - major band.
I I
-1--1-----------1----------1-----------1----------1-----------1----------1-----------1-----------------.~---------'
~.00£+02 • • •
'5.00£+02 •
-11.6
0
• •
165 AF
• •• 3 II
2 4.2 211 32 211 3. ~3
2 ••
l\J
IW
~
r-..
~
'-'
~
oM
~
~.00E+02 •
3.00£+02 •
2.01)£+02 •
\ .00£+02 •
CIT 400 • •• • 2 2 :5 3 :5
• • • 2 • •
• ••
• • • •
• • • •
15,300 •• •
•
•
• • •••
•• • •• •
• • •
•
•
4 •
·22·'5.3 ••
• 2.23.2 2 ••
2·22 ••••••
··2.22 ••
3
1
23 22 2 • 2 1
2·2 • 1
2 •••• 2 • 1
O.OOE+OO •~. *
1_1 --__ 1_--------1_-----_--1_--__--__ 1 --__--1 --__ 1 - 1 1___ 1 ' __ 1_1
0.00£+00 2./lOE+01 4.80E+01 7.20£+01 Q.bOE+01 1.20£+02
distance (kIn)
Fig. 2.1-18 AgI curtain-convection, curtain 15 kIn downwind and 300 mb above
valley floor and top of convective instability at 400 mb above valley floor.
Sounding type - major band.
* •
i-I---------I---------I---------I---------.--------~---------.----------1-------------
~.OOE+OZ • * *
distance (km) •
3
2. '5 **2 2 **2 q
2 2 *3 3 *36 •
*
-16.70
* * * 2 2 2* 2* *2* *] * 4 *.
* * 2 * 2 * 2 * 2 * 2*2 **'5 **
* * * ***2
* * *
2 * * * * *
2 * * *
2 * *
* **
* * *
** * *
2 *
*
*
*
* *
*
* **
*
* *
* *
* *
CIT 450 *
* *
•* *
*
~.00E+02 • 15,300 u
* O.OOE+OO ~1_1~--_1 __-- 1 --*-_--1 1 --1 ----1 1 * 1___ 1 1_1
O.OOE+OO 2.QOE+01 4.80E+01 7.20E+01 Q.60E+01 1.20E+02
1.00E+02 •
2.00E+02 •
5.00E+02 •
4.00[+02 •
,.--..
~
\.-J
~
.,-i
]
I:\:)
I
CJJ
C.1l
Fig. 2.1-19 AgI curtain-convection, curtain 15 km downwind and 300 mb above
valley floor, top of convective instability 450 mb above valley floor.
Sounding type - major band.
1-.---------.---------.---------.---------.---------.---------.---------.----------.---------
6.00E+02 - • * ..
».00£+02 - CIT 500
r-..
~. ~.00E+02 -
\.....J I " I • ~ ..t::. I •
bL)
.r-4 j.OOE+02 - 15,300 •• ~
l'V
I .2.00£+02 - VJ
Q)
1.00E+02 -
O.OOE+OO -l_l l l I I ~ I I ~I---__----I---------I---------I-I
O.OOE+OO 2.40E+01 4.80E+01 1.20E+01 q.~OE+01 1.20E+02
distance (km)
Fig. 2.1-20 AgI curtain-convection, curtain 15 km downwind and 300 mb above
valley floor, top of convective instability 500 mb above valley floor.
Sounding type - major band.
* •
I-I---------I---------I---------I--------- --------~---------I--------~-------__.___ ------------
6.00[+02 • * *
5.00E+02 -
4 *
*22**5*3 ***
* 2**3 *2 2 * *
2**22* 2 ** 23 22 2 * *
**2*22 2***2 * * 2
***** 2 * **
-l1.ao
* * * 3 II
CIT 400 * ** * 2 2 3 3 3 2 4 *2 24 32 24 3* B
* * * 2 * * 2 * *
* **
* * * **
* *
15,300 ** * * * *
* * * *
O.OOE+OO •I_I I_~ I I - I I I 1 1___ 1 1_1
O.OOE+OO 2.40[+01 11.60[+01 7.20E+Ol Q.bOE+Ol 1.20[+02
1.00E+02 •
r--- .\.00[+02 - ~
'-'
~
~
.~ 3.00E+02 •
Q) ..c::
l:V
Iw
-..J 2.00E+02 •
distance (kIn)
Fig. 2.1-21
valley floor,
SOWlding type
C02 curtain-convection, curtain 15 kIn downwind and 300 mb
top of convective instability 400 mb above valley floor.
- major band.
above
-------------------------------------------------------------------------------------- I 1 I 1 1 1
6,OOE+02 •• • • •
• • •
0.1 AF*
NAWC
* (SOME TRAJECTORIES f\OT RUN
LONG Ef\OUGH TO REACH GROUND)
• • o ,OOE+OO - ~ •
1_1 --_1 -- 1 1 1-- --1 --- I ~I I I_ I_I
O,OOE+OO 2.QOE+01 a,AOE+OI 7,20E+0\ q.bOE+O\ 1.20E+02
1,OOE+02 -
5,OOE+02 ·
r-..
'@ '--' ",OOE+02 · CIT 350 -8.9°
I • * * *;0 .. •
~ I ~. 2 ••
'"Sn I • • 2 2 2 2 • ? • •
oM I • • • 2 • CJ) .r:: 3.00E+02 · • • • • •• 2. 2 2.
I • • • • •
t-:l I • • • • • I 1 • VJ
00 I • •
2,00[+02 · ••
distance (kIn)
Fig. 2.1-22 AgI point source-convection, ground source 20 kIn downwind and
40 rob above valley floor, top of convective instability 350 rob above valley
floor. Sounding type - Cl.
,-a---------,---------.---------,---------i---------,---------.---------a---------.---------r----------I--I
6.00E+02 .. • " ...
3.00E+02 ..
-13.9°
" "*
•
" "
"
I11
2 3 • I
* 2. 2 l' ..
2 " 2 • •• 3
" 2
•
" "
"
* 2 *2
"
"
"
""
"
* "
12 AF*
NAWC
*(SOME TRAJECTORIES NOT RUN
ENOUGH TIME TO REACH GROUND)
•
• " .
*•
2
•
*
•
• •
2 2 *3
, 2 2 2 A AA •
2.00E+02 ..
1.00E+02 ..
O. OOE +110 ..
5.00E+02 ..
II
r--'\ . I
~ I
~ 4.00E+02 ..
~
'r-!
]
tv
I
CJJ
(,0
1_1 1 1 1 1 1 1 1 1___ I I_I
0.00£+00 2.QOE+Ol 4.60E+Ol 1.20E+Ol q.&OE+Ol 1.20E+02
distance (km)
Fig. 2.1-23 AgI point source-convection, ground source 20 km downwind and
40 mb above valley floor, top of convective instability 400 mb above valley
floor. Sounding type Cl.
1-1---------.---------1---------1---------1---------.---------I---------.---------.---------~--------s_I
6.I)OE+02 - • • .-
5.0 OE +02 - -17.5°
~ I CIT 450 ~. 2· ••
~ 1 • • • • :3 • • 2
'--' 1 • • • • • 2. • :3 :3 :3 q
~ 4.00E+02 - • • • • • -
-Sn 1 • •
1 • • • 'M
(1) 1 • • • • .J:: 1 • • • • • •
3.00E+02 - • • •
I • • • • •
1 • •
I:\:l
I~
2.00E+02 - 0
1.00E+02 -
O.OOE+OO -1_1 1 1 1 1 1 I ~I I___ 1 1_1
O.OOE+UO 2.QOE+O\ Q.80E+O\ 7.20E+01 q.&OE+O\ \.20E+02
distance (km)
Fig. 2.1-24 AgI point source-convection, ground source 20 km downwind and
40 mb above valley floor, top of convective instability at 450 mb. Sounding
type Cl.
Table 2.1-6. Summary of unstable Guide runs.
Coldest Precip-
Nucleating CrT Temperature itation
Figure PET Method X (km) Z (rnb) (rnb) -(DC) (acre feet)
Curtain:
2.1-17 Band TB-1 15 300 350 - 8.8
2.1-18 Band TB-1 15 300 400 -11.6 165
2.1-19 Band TB-1 15 300 450 -16.7
2.1-20 Band TB-1 15 300 500 -21. 6
Ground:
2.1-22
2.1-23
2.1-24
Cell
Cell
Cell
NAWC
NAWC
NAWC
20
20
20
40
40
40
2-41
350
400
450
3*
* SIGNATURE
* *
*
*
~l---------I---------I---------I---------I---------I---------~-------~--------~---------~--------~
5.115E-01 - J FALLOUT TRAJECTORIES
I * *.
1
1
I .
3.1IE-Ol •
,-...
! I.Cl1E.OI·
~O
'M
(1) ...c: l.l5E-00·
'\. FOLSOM D LAKE TAHJE
I.\:l
I
I*.\":l
-1.5lE-0 1 -
-S.lbE-Ol -
-5.00E-01 -l_l* I * * * * q • I_~ I I l I 1 1 1 1_1
o.OOt_oo 2.QOF+01 Q.AOE_OI 7.20E_01 q.~Ol.OI 1.20E.02
distance (kIn)
Fig. 2.1-25 AgI point source-convection, xy plot. Sounding type C1.
in the most active portion of bands). There is a suggestion
that C02 seeding might be more effective than AgI in developing
a dynamic response.
2.2 Hypotheses and Response Variates
In consideration of the existing network, and of past
successes and failures in observing individual bands and cells,
it would appear that the use of a band as a unit of observation
is feasible, while that of a cell is problematical, and possible
only if very precise aerial seeding is practiced. In the ground
seeding scenario, the lack of control precludes the individual
cell approach and a three-hour (or longer) period of area-wide
cell seeding by a ground generator network is recommended.
Table 2.2-1 summarizes briefly the status of SCPP with
respect to observations along the links in the chain from
curtain to ground. In the case of ground generators the sampling
of the "curtain", in this case a psuedo-curtain generated
from a line of generators and being entrained into the inflow
zone, the measurements are far from precise. There are also
great differences between convection types. Specifically,
the observation of interactions between the curtain's seeding
material and the cloud is much more easily observed in the
case of cells than in the case of bands. Individual cells
can be clearly observed and penetrated with precision, whereas
cells in bands are obscured by cloud and difficult to find.
It is no wonder that situations of high water content have
been more frequently observed in cells than in bands; this
condition reflects ease of observation.
Aerial observations in the past have tended to concentrate
in the -50 to -100 C zone, where most of the calibration seeding
2-43
Table 2.2-1. Links in seedin~ chain.
Link Name
1. Curtain
2. Fallout
(aloft)
3. Precipitation
on the ground
4. DYnamic effects
5. DYnamic effects
(ground)
How Measured (both directly
and indirectly)
• Wyoming AC
• Radar
• Sounding data and Guide
Model
• Sounding data and Guide
• Unexplored radar
possibilities
• Precip. gauge network
• Ground level microphysics
• Radar for horizontal spread
and/or top rise
• 15 min. precipe gauges
• 15 min. meso-net
• Ground microphysics
How Precis~?
Moderate tJ good
Dispersion aspects fair
to poor
Fair to moderate
MOderate to good
Fair
Good for bands
Fair to poor for other
convection
has been done. The HUBBLE runs demonstrate the importance
of making observations through an extended vertical profile.
The profile should penetrate individual cells so that microphysical
properties at various levels in a given cell can be compared.
This requires flight procedures different from the one level
tracking by means of a "homing pigeon" device which has been
used appropriately for the seeding calibration I'uns. When
this approach is used in a convective draft the seeding material
and associated microphysical particles moving Lpward past
the sampling level is being observed in seeded eells, and
its interpretation is thus complicated.
2-44
At this point it should be reiterated that a shallower
curtain is recommended. In fact, the "curtain" generated
after a period of an hour by a moving point source would be
sufficiently deep for proper entrainment into the inflow zone.
This procedure would reduce the amount of redundant seeding
material and sharpen the analysis. Under this plan the seeding
would normally be conducted at temperature levels warmer than
the -50 to -lOoC zone, so that seeding would only occur where
the nucleant is entrained. It would be virtually impossible
to conduct this type of seeding by injecting the material
directly into selected cells within a band (although it could
conceivably be done in widely spaced individual cells). The
procedure for bands would be directed toward treating the
entire width of the band along a sector that is tens of kilometers
long. In the case of ground seeding the line of continuous
generators would seed numerous cells over an extended area.
In either case, aerial sampling (which could not possibly
cover the entire treated area) would concentrate on convection
that is predicted to be well targetted.
Table 2.2-2 lists in summary form the various response
variates, comparison analyses, and hypothesized effects of
seeding for major bands. It is assumed that the seeding would
be carried out as discussed above, with three-way randomization:
AgI, low C02, and placebo.
Table 2.2-3 summarizes similar information for the scenario:
cells, ground generator seeding. Here it will be noted, the
aerial observations and radar can define individual cell behavior,
however, the ground level observations (with the probable
exception of microphysical) define only observations in a
treated and a comparison area.
2-45
Table 2.2-2. Major bands, modified curtain seedir.g.
Response Variates
Vertical profiles of
LWC, IWC, ~VC, vertical
velocity across seeded
segment. SlITIilar
observations outside
of treated segment.
Radar (mostly for
fallout echoes)
Echo dBz, top, width
Ground level precip,
pressure, wind per­turbation
(15 min.
resolution) and
microphysical
perturbation
Comparison Analyses
• Inside/outside of
treated band segment
• Seed/placebo in
treated segment
• Inside/outside of
treated band segment
o Seed/placebo
• Inside/outside of
treated band segment
• Seed/placebo
2-46
Hypothesized Effects of
Seeding
• Model predictions using
depth, ur:draft, cloud
base, mixing ratio, wind
shear, ard other
parameters.
• Top rise
• Band broadening
• Hore inte:nse perturbation
• Broader perturbation
• Crystal r~bit change
• Increase in particle
concentration
• Decrease in particle
size
Table 2.2-3. Cells, ground generator seeding.
Response Variates
Vertical profiles of
LWC, IWC, PWC, vertical
velocity in cells.
Periods covered
2-3600 sec.
Radar (mostly tracking
treated cells)
Echo dBz, top, width
Ground level precip.
and microphysical
perturbations
COmparison Analyses
• Inside/outside of
selected (obs.)
treated cells
• Seed/placebo
• Inside/outside of
cells
• Seed/placebo
• Inside/outside of
treated cells area
• Seed/placebo
2-47
Hypothesized Effects of
Seeding
• Model predictions using
depth,-updraft, cloud
base mixing ratio, wind
shear, and other
parameters
• Top rise*
• Cell broadening*
* collectively and
individually
• Hore intense area
precip
• LonRer duration area .:> precip
• Change in crystal habit
• Increase in particle
concentration
• Decrease in particle
size
2.3 Seeding Modes and Procedures
2.3.1 Major Bands. It is considered feasible to
seed major bands with aircraft using at least three seeding
systems: dry ice curtains, silver iodide curtains produced
by dropping flares, and silver iodide line seedin~ produced
by using acetone generators similar to that used in Santa
Barbara II, Phase II. In addition, major bands with roots
of convection reaching to foothill elevations are deen~d seedable
with ground generation techniques (pyrotechnics or acetone
generators as was performed in the CENSARE project in the
Sierra Nevada). As will be documented later, it is fel; desirable
to maintain a three-way randomization scheme testing C02'
silver iodide, and placebo seeding of major bands.
The primary consideration in terms of where to seed in
major bands becomes a question of where liquid water il; routinely
produced. Analyses of SCPP data and the earlier DRI flights
in the SCPP area, and a comparison of Sheridan versus Freshpond
rawinsonde data with respect to water saturation all suggests
that liquid water is generated in the foothill region of the
Sierra Nevada during winter storm periods. Additionally,
regions of liquid water with low ice crystal concentrations
appear to be frequently related to developing or new growth
convection towers. Limited analysis of SCPP data f)r a major
band (Moore, et al., 1980) indicate in one case tha: the major
updraft region in higher liquid water contents were to be
found a t the rear of the major band (see Figure 2.3··1). There
is a suggestion that other major bands may be froat feeders
instead (SCPP analysis conference, Oct. 21-23, 1980), although
there has been the suggestion of conversion of froat feeding
bands to back feeding bands as the band encounters the foothill
region of the Sierra Nevada. Additional studies are warranted
2-48
6e+4
6e+2
6e
~
=> BAND
MOTION
-5°e -- J
ooe
(&)
7 7 71/77777777717777
~---30 -50 km -----~
Fig. 2.3-1 Convective band schematic. Wind barbs shown at 2, 4,
and 6 km, respectively, are band relative. Speed is in knots.
Schematic is representative of a band in the valley.
2-49
to develop the capability of determining the location of this
updraft region within major bands hopefully through €'xamination
of radar returns (conventional and/or doppler), although direct
measurement of this region may be necessary either by the
seeding aircraft equipped with a JW instrument or a supplemental
cloud physics aircraft.
In the case of dry ice or silver iodide curtaj.n releases
in major bands, it is proposed that these curtains bE! ini tia ted
at the -10o C level. Figure 2.3-2 provides a mean sounding
for major band occurrences as documented by Electronic Techniques
Inc. (ETI). On this mean sounding the -10o C levl~l is near
12,SOO feet MSL. If the dry ice and flares drop approximately
4000 feet, then a curtain should be produced from ~bout -10oC
to -3 0 C level (approximately 8S00 feet MSL). 11 the case
of an acetone generated line source it is proposed that such
a source be genera ted at ei ther the OoC or -SoC leve:. depending
upon terrain clearance considerations and possibly icing consider­ations
in flying for long durations within convection bands
of this type. The advantage, or course, of flying at the
OoC level is that ice should not accumulate on the aircraft.
For the mean sounding, this would either be 7S00 feet MSL
or 10,000 feet MSL. Assuming an average updraft strength
of 1 m/ sec, the m'a terial would rise from the OoC to the -10oC
level in about 13 minutes and from the -SoC to tile -10oC in
about 8 minutes.
Seeding of major bands would commence in secto:s of baads
as they move eastward from the Sacrmento Valley area into
the SCPP experimental area. It is proposed that seed:.ng commence
over approximately the 1000 foot contour in the footllill region
and terminate at the 4000 foot contour which rep:resents the
western boundary of the SCPP experimental area. The lower
2-S0
l:\:)
I
U1
f--l
f:>0
WINO SCALE
400
700' 7" / 7" / / / / / ~ / / / / / /. / 4 J 1700
800/ / / / / / / / / 7{\ / / / / / / J /+ V leoo
900 / / 7' 7' / / / / / /) \: / / / / / / t % • / I
I000 ,./ ,./ 7 / 7 / 7 / 7 " 7 " 7 " 7 " 7A' 7 / 7 / 7 / 7 / 7 / >Vr f 7 / ~ 711000
Fig. 2.3-2 Mean sounding for the 1980 forecaster designated major band echo type (PET).
region that this seeding would embrace is favored because
it is likely that the additional orographic indueed uplift
will enhance the production of supercooled water and thus
the seedability within this zone. Seeding further away into
the valley itself would compound the targeting of mi~rophysical
seeding effects in the SCPP experimental area. Seeding past
the 4000 foot contour is not deemed as warranted since this
region appears to be one that is experiencing increased production
of ice and reduced liquid water contents (based upon SCPP
and earlier DRI flights). This procedure will 8.1so allow
pre-seeding alert of the seeding aircraft crew since major
bands normally travel across the valley and some pre-seeding
sampling of the band over the Sacramento Valley.
It is assumed that the GUIDE model would be available
for real time decision making and that it will l>e used to
assist in targeting of the microphysically induced effects
of seeding either with dry ice or silver iodide curtains such
that the mid-point of these curtains would be t.argeted to
pass over the Central Sierra Snow Lab. This apprc,ach should
also produce microphysical effects in the data dense instru­mentation
network along 1-80. As a part of the C:rUIDE model
calculations, consideration is given to the backing and then
veering of the steering level winds as the air mass approaches
and then goes over the Sierra Nevada barrier. In this regard,
it is conceivable that the GUIDE model might suggest seeding
some distance further south than might initially be expected
by examining say the 700 mb wind flow at the Sheridan rawinsonde
observation site. It is proposed that seeding would be conducted
along a 30 km sector of the band and that this sector would
be, as mentioned before, centered to affect the Central Sierra
Snow Lab. Continuous seeding in the updraft region of the
major bands is proposed from the initiation point of seeding;
2-52
i.e., the 1000 foot to the 4000 foot contour level or a distance
of about 45 km. It may be advantageous to develop some termination
criteria such that a major band that is initially seeded is
discontinued whenever the maximum dBz level along the band
in the SCPP vicinity drops below some minimum dBz value such
as 20 dBz.
The horizontal pattern produced by aerial seeding of
major bands would appear to be a zig zag pattern if it were
plotted out schematically. The expansion in the curtain or
line generated curtain would of course expand with time.
As Vardiman has pointed out, the overlapping of curtains is
assisted by speed shear between the top and base of the curtain.
A seeding aircraft flying at 150 knots would produce curtains
within bands along a 30 km seeded sector such that without
any speed shear the plumes would probably not merge over the
SCPP experimental area if the horizontal dispersion in bands
was approximately 1 mjsec or less. With a 1 km curtain and
seeding over Folsom with a 5 kt speed shear, the plumes would
overlap in the 4000 foot contour region only if a new seeding
curtain was generated approximately every five minutes - not
practical at 150 kts over a 30 km sector which occupies approx­imately
10 minutes. If the shear were ten knots, then plumes
would overlap with curtains generated every 16.7 minutes ­within
the time available. As seeding progresses up the barrier
as a band moves eastward, the plumes would become less prone
to overlap due to the shorter distances between release and
the 4000 foot contour level. They would tend, however, to
overlap further downwind over the upper part of the barrier.
Figure 2.3-3 provides a schematic of what the flight tracks
might resemble for a typical operation.
2-53
tv
I
CJ1
w::.
YUBA CITY
N+
~
o JACkSON
o 5 10 2,0 '3,0 4:' kiLOMETERS
I , , 20
? 5I 1I0 I NAUTiCAL MILES
Fig. 2.3-3 Possible seeding pattern in a major band.
The GUIDE model has been utilized to simuate the micro­physical
seeding effects of a major band with varying seeding
modes and input sounding data. Dry ice, silver iodide curtains,
and silver iodide line source releases were all simulated
for varying depths of convection and h€ights of seeding.
Figures 2.1-6 through 2.1-25 contain these plots. It appears
seeding with all three generating systems would pr~uce seeding
effects on the upwind side of the barrier. By using the GUIDE
model to target the microphysical effects centered on Central
Sierra Snow Lab, taking into consideration the backing of
winds with progression up the barrier, the 30 km long sector
should allow seeding effects to be detected along the 180
network as well. Any additional dynamically produced seeding
effects are anticipated to occur to the right of the steering
level flow (as in NAWC's Interim Report No.2) such that these
effects should be detectable in the central and southern portions
of the SCPP experimental area.
A cost comparison of dry ice and the two methods of silver
iodide seeding is as follows: Assuming an average band movement
equivalent to approximately one half of the 700 mb velocity
noted by ETI in bands would be 10 m/sec. The total time of
seeding between initiation of seeding in the 1000 foot contour
region to termination at the 4000 foot contour should occupy
(about 45 km east of initiation) 75 minutes. Allowing 7 minutes
seeding time and 3 minute turns, this translates into approximately
seven seeding curtains when flying at 150 knots. During this
period, 21 kg of dry ice would be used (at the seeding rate
of 100 g/km), 420 flares (spaced every 500 m), or 2-3/4 gallons
of 2% silver iodide solution (or about 150 g of AgI) would
also be used. The approximate cost of seeding one major band
for these three modes would be $30, $8400 ($20/each), and
$100. The output of the acetone generation system could easily
2-55
be doubled if desired in terms of seeding rate considerations
by burning two generators simultaneously.
2.3.2 Ground Seeding of Convective Cells. The utility
of ground generators to seed convective cells (either Cl or
C2) depends upon a number of factors. Primary considerations
include the relative instability of the atmosphere, the temperature
of the air mass, and prevailing wind directions and speeds.
Of perhaps primary importance is the stability of the atmosphere.
Under stable conditions the entrainment of effluent from ground
generators into active growth regions may be restricted.
In the case of PETS Cl and C2 , we know from ETI'~ work that
these PETS typically occur post-frontally and are corrEspondingly
normally unstable in character. Figure 2.3-4 (j:rom ETI I S
Interim Progress Report dated July 1980) illustrates the average
conditions. This figure indicates a region of instability
beginning near 1000 m for C2 and near 500 m for C1 ca.ses.
It is assumed that any seeding from the ground would
be performed with silver iodide dispensers. Dry ice does
not ofer a tractable capability from the ground and organic
generating systems are still in the prototype stage (If develop­me
n t. Due to the we 11 known temperature dependency of silver
iodide in"terms of the production of active ice nuclE~i, seeding
material must be transported aloft from the surface to reach
effective levels starting near the -5 to -6o C level. From
Figures 2.3-5 and 2.3-6 (also taken from ETI's Inter:Lm Report)
i tis seentha t the - 50C 1eve 1 for C1 's is near :~ 5 0 0 mand
2800 m for C2 's. Radar climatologies of the SCPP have cemonstrated
a tendency for cells to form over the foothill location (approxi­rna
tely 300-1000 m) and to move up the barrier and weaken (Suther­land
et al., 1978).
2-56
9
CI C2 CS MB EB AW/OR
E--IOOK~
7
2
8
3
6
.­I
(!)
w 4
I
O....J...__......1- ..I.-__--L__..I.-_-L__..I.- _
Fig. 2.3-4 Mean vertical profiles of equivalent potential temperature
(solid lines) displayed with the low level maximum 8e isotherms (dashed
line) by forecaster designated PET in 1980.
2-57
/Of) f)0 ~ ,," ,,0
l.'V
I
01
OJ
700
800/ , , , , / / ,..... / \ / / / / / / / t /1 j/ 1800 > > > > • • • c: • • • • • • > >
900
/ / / / / / / / / .X\/ / / / / / ~'OOO 1000 • • • • • • • • • • • > •
Fig. 2.3-5 Mean sounding for the 1980 forecaster designated cellular echo type (el).
400
WIND SCALE
ri> ",f) ",0 'Of) ~ .,f) f)0 ",f) ",0
~
I
CJ1
(!) 700///,/././.AL-A/././././,/,/i.XKl700
800/ £ £ £ £ £ , ,
7' \: 7"" 7
,
7
,
7
, , , , , , , , , / t /+ j/ 1800 7 7 7
900
1000/ ,L ,L ,L ,L >L ,L >L >L >L l;r >L >L >L >L >L l4ilJ1000
Fig. 2.3-6 ~1ean sounding for the 1980 forecaster designated cellular echo type (C2).
Based upon the above characteristics the follovling ground
seeding mode for convective cells is recommended. Multiple
lines of remotely controlled ground generators should be estab­lished
generally in different elevation zones. Fo.r example,
three lines of generators could be installed along the 300,
900, and 1500 m contours. Such an array is depicted in Figure
2.3-7. Spacing between generators would be on the order of
5 km. The lines would be situated such that when prevailing
wind flows are considered with C1 and C2 types, the expected
fallout of augmented precipitation would occur over the northern
portions of the SCPP experimental area. The Guide Hlodel would
be utilized to develop a climatology of this area for siting
guidance. The rationale for multiple lines arises f)'om expected
deviations on individual cases from the mean soundings ]~epresented
in Figures 2.3-5 and 2.3-6. If the level of convective instability
is higher than the norm, higher elevation genera":ors could
be utilized if they are indicated to be above an~T low level
inversions or isothermal regions provided wind speeds are
not excessive. The Guide model should be made available for
real-time seeding guidance to assist in targetting the effects
of seeding.
2.4 Estimate of Natural and Augmented Precipitation and Frequency
of Occurence
2.4.1 Major Bands. Data furnished by Atmospherics
Inc. (AI) provides a mean hourly precipitation rate oj' 2.2 mm h- 1
for the periods when major bands were the dominate echo in
the American River Basin during the 1979 and 1980 ob:;erva tional
seasons. These data are for non-zero precipitat:Lon cases.
Information from ETI indicate a mean duration of 3.43 hours
for major bands in the American Rivee Basin from radar data
(one-third of the 1977-78 season and all of the 1978-79 and
2-60
t\J
I
O'l
......
YUBA CITY
N+
o JACKSON
o 15 10 2,0 3,0 "p KILOMETERS
I , I 20
o, 1,5 1I0 I NAUTICAL MIL!S
Fig. 2.3-7 Possible remote controlled ground generator network.
....~ .
1979-80 data). Consequently, a mean natural prl~cipitation
amount for a major band occurrence is calculated to be 7.5 mm.
The seeding potential according to Sections 2.1 and 2.2 is
100% or more increase in about one third of the ~ases. The
band precipitation to be measured as a response variate is
probably about 7.5 mm per band, lasting about one hour, with
a coefficient of variation of about 1.
An examination of PET occurences during tne 1979 and
1980 opertional periods indicates 10 major bands Ln 1979 and
21 in 1980. If an average of 15 bands for the ~~-1/2 months
sampled is considered, then a total estimate of the number
of major bands per operational season would be 50 per year
(twice the number for day/night operations and 1.67 times
to account for a full season of operations).
2.4.2 Convective Cells. Data furnishei by AI for
C1 and C2 PETS combined indica te a mean hourly p::-ecipi ta tion
rate of 1.64 mm hr- 1 for the periods when C1 and C2 were the
dominant echo during the 1979 and 1980 seasons (these data
exclude zero precipitation events). Informati)n supplied
by ETI indicates a mean duration for C1 and C2 P.~TS combined
of 6.04 hours. Consequently, a mean natural pr('~cipitation
amount for C1 and C2 occurrence is calculated to be 9.91 mm.
The seeding potential according to sections 2.1 and 2.2 is
100% or more increase in about one third of the cases.
I t is estimated that 29 separate cases of C1 and C2 occurred
during the 1979 and 1980 observational seasons. If we use
an average of 15 cases per year, double that number :Eor day/night
operations, and mUltiply by 1.67, we reach an estimate of
50 cases per operational season.
2-62
2.5 Gaps in Knowledge
We need a better understanding of where the updraft regions
are located within major bands. It is hypothesized that these
regions will contain the highest liquid water contents and
will therefore represent the desired seeding regions. Techniques
of identifying these regions in a real-time mode are needed
(for both day and night operations). Remote sensing techniques
applicable to this problem should be considered if at all
feasible.
If ground generators are to be considered in an exploratory
phase of the experiment, then additional information on required
purge times is needed. This information will be required
to separate experimental units such that not-seed events remain
uncontaminated.
2-63
3. TASK FORCES 6 AND 9
Once the decision had been reached to suspend most field
activities on the SCPP for the 1980-81 season in order to
concentrate on detailed data analysis, nine different task
forces were organized to perform this analysis work. A lead
scientist was designated within each task force to coordinate
the analysis and reporting of results. NAWC was assigned
two task forces in which lead scientists were provided - Task
Force 6 - Robert D. Elliott "What is the opportuni ty for dispersing
seeding material by PETS and what changes in amount, duration,
intensity, and distribution of precipitation would be produced
by seeding as a function of PETS?" and Task Force 9 - John
A. Flueck "What are '~he major statistical components of an
exploratory experiment as a function of PETS?" Team members
on these two task forces are provided below.
Task Force 6
Brooks Martner (UW)
John Marwitz (UW)
Bill Moninger (NOAA)
John Flueck (FA)
Jim Humphries (Bureau)
Mark Solak (Bureau)
Rick Stone (DRI)
Rand Allan (AI)
Ron Stewart (UW)
Task Force 9
Robert Elliott (NAWC)
Don Griffith (NAWC)
Larry Vardiman (Bureau)
John Marwitz (UW)
Owen Rhea (ETI)
Key: AI
Bureau ­DRI
ETI
FA
NAWC ­NOAA
­UW
Atmospheric Inc.
Bureau of Reclamation
Desert Research Institute
Electronic Techniques Inc.
Flueck Associates
North American Weather Consultants
National Oceanic and Atmospheric Administration
University of Wyoming
3-1
Final reports were generated by each of the task forces
for presentation at a SCPP analysis conference held in Denver
on May 19-20, 1981. Section 3.1 and 3.2 provide these final
reports for Task Forces 6 and 9, respectively.
3.1 Task Force 6 - Transport and Diffusion
Cloud seeding technology attempts to change the water
balance of a natural precipitation system in sueh a way as
to alter or enhance the precipitation therefrom in a desirable
manner. In order to properly accomplish this, it is necessary
to 1) understand in some physical detail the natural process,
and 2) understand the modification of it. The scope of this
understanding must extend well beyond the mere detection of
a potential for modification. It must include the physical
details of the transport and diffusion of the seeding agent,
the nucleation process, the growth of ice crystals within
a seeding II signature", and· the subsequent fallout of precipi tation
size ice crystals to the ground. In this section a numerical
"Guide" model is discussed that embodies within itself these
details, organized so as to provide a practical aid i~ operations
and evaluation.
The basic concept of water balance in an orographic cloud
in which there is no convection is presented schematically
in Figure 3.1-1. The cloud is shown extending upwind in an
"area-wide" mode; however, the cloud on many occasions appears
only over the mountain. A set of streamflow channels indicates
the speed-up of air flow normal to the barrier, while the
sloping channels depict the fallout of an average particle
on its growth and descent from initial nucleation at high
levels in the cloud. Some significant variations from these
idealized patterns will be discussed later. A1BO shown by
3-2
FALLOUT CHANNELS
I
120
I
90
I
60
I
-30 o 30
en
"-J
LIJ
ZZ~
J:
() --.
3t
0 --. -J u..
DISTANCE (km) ~
Fig. 3.1-1 Orographic cloud concepts.
C
D
LW ­SD
condensation zone
depletion zone
liquid water accumulation zone
saturation deficit zone
dashed lines is the pattern of concentration of supercooled
liquid water, with a peak near the ground at the crest. Several
numerical models support this type of pattern which results
as the upslope advection of low level condensate and its production
by lifting exceeds its depletion by growing precipitation
particles in this region. Limited observational evidence
supports this for SCPP at present.
A numerical version of this conceptual model was constructed,
using the same cloud physics and treatment of fallout as was
used in Guide. A run was first made for a typical orographic
cloud, then the nuclei content at cloud top level was enhanced
3-3
five-fold. to simulate a broadscale seeding effect. The results
are summarized schematically in Figure 3.1-2. ~he figures
at the top of the fallout channels are the nuclei concentration
enhancement factor for that channel. The ones at the bottom
are the precipi ta tin enhancement in rnrn hr- 1 for tlla t channel.
In addition, seeding reduced particle size so that the channels
were shifted downwind at low levels as indicated by the arrow.
This is the "redistribution" effect of the seeding which under
extreme conditions leads to overseeding (i.e., that is to
the downwind transport of ice particles to the lee side evaporation
zone). This conceptual model provides a basis for a seeding
enhancement potential. However, there is a long stip between
establishing such a potential on the basis of orog::-aphic scale
water balance considerations and the implementa-~ion of the
required enhancement of the nuclei concentration. This latter
involves both the transport and diffusion of finite sized
emissions of nucleant from a fast-moving aerial sou:rce.
Fig. 3.1-2 Precipitation change in fallout columns with
seeding (rom hr-1).
3-4
Starting with the problem of transport, it is necessary
to know the air flow at any level over the barrier. In practice,
the main source of information concerning the airflow in SCPP
is the upwind sounding taken every three hours at Sheridan.
The air flow undergoes significant modification on passage
over the barrier. First, there is the acceleration of the
normal flow over the barrier as depicted in Figure 3.1-1.
In a neutral atmosphere potential flow theory calls for an
exponentially decaying perturbation to extend upward to infinity.
In the real atmosphere neutral conditions disappear at the
tropopause with a stable "lid" above. In winter storms the
lid where the flow becomes flat appears at about the 400 mb
level. But if the atmosphere beneath is stable, as in the
"stable orographic" cloud case, the flow crest tilts upwind
aloft from the terrain crest in the manner shown in Figure 3.1-3
This is called for in theory and is supported by analyses
of aerial observations of the King Air and the sounding cross
o
I I
30 60
DISTANCE (km)
I
90
I
120
Fig. 3.1-3 Typical stable flow pattern.
3-5
section analyses (Sheridan, Freshpond, Reno) by ETI. More
analyses may reveal variations with the degree of stability
and the strength of the basic normal flow, as is expected
also from theory.
In addition to the variations in the normal flow over
the barrier, there are quite severe variations i1 the flow
parallel to the barrier. The existence of a foothill "blocking
flow" directed north-northwestward parallel to the Sierra
Nevada range has been known for many years. CENSARE Jbservations
showed the development of a high surface pressure "dam" along
the foothills that deflects approaching air northwarj. Analyses
·of SCPP observations, including sounding cross sections, paired
soundings, and King Air data indicate that this perturbation
diminishes up the slope as indicated in Figure 3.1-4.
All of these patterns have been incorporated into the
transport module of the Guide model, where the basic input
is the Sheridan sounding winds. Note the dead layer in the
figure, where the normal component is nil. The depth of this
1-----__
I
12.0
I
90
I I
30 60
DISTANCE (km)
. 0
2-·- -­3-_
" ~ 4---
Fig. 3.1-4 Typical V component pattern.
3-6
layer is frequently one kilometer or more well ahead of the
front where area-wide and stable orographic clouds prevail,
and decreases to near zero post-frontally.
The input to Guide, for the orographic cloud's distribution
of liquid water, is a modified version of the pattern shown
in Figure 3.1-1. The modification is based upon spot observa­tions,
the pattern remaining the same with the amplitude adjusted
to fit the observations. In the absence of any observations
a cloud climatology would be used. It should be pointed out
that an orographic cloud can also be present when convection
exists, but is usually of limited depth, except in the case
of embedded convection. In late post-frontal cells it may
be non-existent.
Diffusion of particles on the scale of a seeding curtain
in theSCPP area have been studied in two ways: 1) in the
growth in width of a tracked seeding signature, where the
concentration of artificially produced small ice crystals
are measured over periods of up to an hour, and 2) the turbulence
spectra derived from measurement made by the King Air's horizontal
and vertical vanes. How these measurements can be used in
a practical way will now be discussed.
A curtain of nucleant is initiated by dropping a series
of pyrotechnic flares along a line. They are spaced out about
250 meters apart and each emits some 20 grams of AgI along
a one kilometer depth. Figure 3.1-5 shows a top view of the
starting configuration. In a second or so each nucleant trail
expands to several times the diameter of the flare, thereafter
dispersion is governed for several minutes by the law y2 = E t 3 ;
i.e., the width Y expands at the 3/2 power of time. E is
3-7
o
o
o
o
o
I' .... '",
I \
I I "\/~- ...... - -/" .......
I \
( \ \/ ) ....._....
....
/
I
I I,
"- .....
x
....
",
\,
/
/
./
START 5 MIN 10 MIN
Fig. 3.1-5 Typical flare expansion (circles) and
center dispersion (arrows).
the energy dissipation rate (measured by the MRI turbulence
meter). The expansion rate decreases when the size exceeds
the Lagrangian scale. This dispersion is depicted by the
circles that expand with time in Figure 3.1-5.
At the same time larger eddies disperse thE! centers of
the nucleant trails as indicated by the arrows in Figure 3.1-5.
The rectangles combine these dispersions and depict the resultant
curtain at different times. Note that within fLve minutes
the individual nucleant trails are starting to overlap.
Figure 3.1-6 shows a typical curtain width against time.
Beyond the first five minutes or so theory and observation
3-8
10
4 -Cl) AI
~ -- -CD CD
E-:r: 103
~
0-~
5 20 80
TI ME (minutes)
Fig. 3.1-6 Typical curtain spread.
indicate that the spread becomes proportional to t 1/ 2 • However,
it is both convenient and reasonable to use a linear rate
of spread shown by the dotted line A-A', that is an average
over the domain of interest to seeding. A faster initial
rate is used up to five minutes after start time. The one-sided
rate of spread varies from a few tenths of a meter per second
under very stable conditions to several meters per second
in convection.
3.1.1 Purpose and General Description of Guide.
The Guide model will ultimately be a numerical model providing
guidance for SCPP seeding operations. It will be used for
planning specific seeding operations so as properly to target
a desired area. It will also be used in the evaluation to
identify predicted areas of effect independently from any
response variates. In a sense, it can serve as a co-variate.
3-9
In its present state it serves to test various assumed seeding
modes under different assumed ambient conditions. These simulated
seeding runs provide insights into the bounds of seeding effects.
The general scheme of the model will now be describ~d.
The model assumes that a seeding "curtain" is produced
in aerial seeding, ei ther by means of droppable ~gI flares,
or dry ice (C02). A typical AgI seeding curtain is formed
by dropping twenty 20-gram pyrotechnic flares per mj.nute (about
100 g (km- 1 ). Curtain lengths employed vary with the situation,
but usually run about 10 km. The dry ice treatment is handled
in an analogous fashion. The depth to where complete evaporation
occurs (for normal size pellets) is about the same as the
burnout depth of AgI flares. The merging into a uniform concen­tration
of nucleant (really ice crystals) is possibly more
rapid.
The initial curtain then mixes with the environment at
rates that are input. This curtain mixing (or dispersion)
reduces concentrations of ice particles in the (~urtain, and
mixes in more liquid water from the environment so that growth
of the particles in the curtain can proceed. Bef~re terminal
velocities become appreciable, the curtain is advected by
the mountain air flow along a stream channel. When such a
curtain is produced and sampled it shows much higher ice crystal
concentration (ICC) and lower liquid water concentration (LWC)
than does its environment. This phenomenon has been called
the "signature". However, such a signature may contain artific­ially
produced fallout from a higher level. The term "signature",
and "curtain" as used herein, will refer only to nuclei or
particles so small that they move with the free air flow.
3-10
After some time, larger particles form and fall from
the signature, continuing to grow by deposition and riming
in the lower "feeder" cloud, until the ground is reached.
Some subcloud precipitation evaporation may occur if cloud
base is high above ground. This is most important in the
lee of the barrier.
Nucleation, ice growth, and precipitation are, of course,
normally occurring naturally at the time a signature is being
produced. However, as discussed in the previous section,
under seedable conditions a reservoir of liquid water accumulates
in the lower portion of an orographic updraft. In a stable
orographic cloud this reservoir would under natural conditions
completely evaporate on the lee side. Seeding taps this reservoir,
producing a detectable perturbation on top of the natural
processes. The complexities introduced when convection is
present will be discussed later.
With dry ice seeding, there is nearly a one step sequence
from signature to fallout. With AgI seeding the signature
is lifted by the orographic updraft to lower temperatures,
where new nuclei are activated. Accordingly, the signature
continues up to the crest, or at least to where dispersion
has reduced its particle concentrations so close to background
that it is undetectable. Fallout from the signature is continuous
along its full extent, once the initial stage is passed, provided
liquid water is available for growth. In the SCPP orographic
cloud setting growth within the signature is normally water
limited.
Falling particles change their terminal velocities as
they grow, and they encounter differing horizontal flows at
different levels. Therefore, their fallout trajectories are
3-11
complex. Guide works as a Lagrangian system for outputing
estimates of both the drift of the signature and of the fallout
trajectories.
The foregoing brief description of the modeled signature
and fallout processes indicates that Guide can be used to
estimate the fallout trajectories and ground irr.pact areas
(footprints) for differing seeding strategies under various
cloud type/wind flow regimes. The model is simple enough
for ultimate real time use. The basic elements of the computing
scheme are illustrated schematically in Figure 3.1-7. The
centerline of a signature (SIG) of width W, depth D, and length
(into the page) L is shown at three successive steps. It
expands as it moves, due to dispersion. At each step, one
fallout (FT) trajectory is computed. With C02 seeding there
would only be the initial nucleation and growth of ice particles
---
==
ORIGINAL
DROP
P2
Fig. 3.1-7 Schematic of signature and fallout steps.
3-12
up to position (1), with sUbsequent fallout of particles to
the ground beyond (1). With AgI a new set of nuclei are activated
(as the temperature falls) in going from position (1) to (2),
and these in turn start their fallout in going beyond (2).
The process repeats between (2) and (3) - etc. The signature
positions and trajectories in the vertical plane are computed
using the wind component normal to the barrier (U) and the
orographic upward component (W). In the horizontal plane
the component parallel to the barrier (V) as well as the U
component is used. The barrier wind flow module is used to
extrapolate from upwind sounding data.
Beyond the initial situation, stepwise growth of ice
particles on nuclei within the SIG depends considerably upon
the stepwise inward diffusion of ambient liquid water content
(LWC). The growth may be. water limited. In fallout (FT),
growth depends upon how the particles in the fallout plume
descend through the feeder cloud below. For accuracy in estimating
particle position, terminal velocities are needed, and this
is related to particle mass and habit.
Mass dependent, mass growth rate formulae were developed
as a convenient means for calculating mass growth during the
time step. There is also a dependence upon temperature in
the case of depositional growth, and upon liquid water content
in the case of riming. Depositional growth formulae were
constructed by adapting various curves of growth. Two different
\
temperature (crystal type) ranges are covered, providing two
options in the program.
The wind shear plays a major role in tilting the curtain.
Characteristic shear values of 10- 3 sec- 1 would result in
the top of a 1 km curtain projecting forward of the base by
3-13
3.6 km in an hour. A review of the geometry and co~sideration
of the fact that fallout from the upper part of the curtain
may make the lower part seem to spread downwind led to the
decision to apply the model separately to the upper and lower
halves of the curtain. The differences in the fallout trajectories
between upper and lower halves is enhanced by d:Lfferences
in terminal velocities. In the colder, upper ha,lf, growth
is inhibited by the excess of nuclei and the fallout trajectories
are flatter. In general, in the orographic case fallout particles
from different parts of the curtain are so sepa~ated that
they fall through a fresh water supply.
3.1.2 Some Guide Runs Illustrating Problems tn Orographic
Seeding. The two PET's, area wide and orographie, will now
be discussed. The principal problem in curtain seeding of
these two types is that diffusion is extremely sma:.l in them.
The curtain expands so slowly that overseeding :Ls the rule
for several hours, and in this time a curtain generated over
the foothi lIs would pass beyond the cres t. Thi::; si tua tion
dictates that seeding be started far upwind over thE~ Sacramento
Va lley, in order to produce the des ired ex tra fa,llou t over
the watershed. This procedure makes targetting morl~ difficul t.
A1 so, i twou 1 d bede sira b 1e t hat the cur t a i n bI~ in i t i ate d
in some cloud form, however thin. This may not be present
over the valley in the case of the strictly orographic cloud,
but would be present always in the area wide PET. Runs were
made using a LWC pattern with a peak concentration of 0.5
gm- 3 near the ground at the crest, and around 0.1 to 0.2 gm- 3
at the curtain level. This would fit with a natural orographic
precipitation efficiency of about 0.80. The dea.d layer was
100 mb thick.
Figure 3.1-8 is a vertical profile of the signature and
fallout from the lower half of a curtain having a point of
3-14
400
500
~ 600
E -7.7
w
0 700 z<
I-CI)
0
N
o 12 24 36 48 60 72 84 96 108 120
X DISTANCE (km)
Fig. 3.1-8 Signature (5), 5 fallout tracks (FT), and one
fallout phnne (FP) , lower half of curtain, orographic cloud.
origin at the 700 mb level, 100 km upwind from the starting
point that is shown in the figure. The opportunity to grow
precipitation size particles does not occur until the 4th
- 600 second step in this figure, and the first precipitation
is intercepted by the ground about 25 km into the section.
Precipitation continues to dribble out up to the crest, and
a little is carried beyond into the Tahoe valley. The total
fallout amounts to 18 acre-feet (AF) for this half curtain,
wi th a peak va I ue loca ted near the asterisk. The· reason the
particles fall almost vertically in the foothills is that
the normal wind component is very low at lower levels. An
average AW and 0 PET sounding was used.
3-15
Also shown is the fallout plume for a given time. This
is the synoptic picture of the plume of particles Wllich would
appear in a radar RHI section to be falling from thE~ signature
at this particular time. Note that this plume is very flat
near the ground as a result of shear effects. The width increases
toward the ground due to dispersion.
The temperature of the lower half of the curtain is only
-6.2°C at the zero point of the section but falls to -7.7°C
near the crest due to orographic lift. At these warm ~~mperatures,
the resulting precipitation particle concentrations (the compu­tations
are based upon the TB-1 temperature curve) are less
than ten per liter. Not all the available water is removed.
At a colder temperature, concentrations are more likely to
remove more of the available water. Figure 3.1-9 shows the
108 120
FT__-I>......
72 84 96
-12.2
24 36 48 60
400
500
SJ 600
E -10.2
wu
700
z
<t
l-
(/)
0 800
N
900
1000
0 12
X DISTANCE (km)
Fig. 3.1-9 Signature (s), and 5 fallout tracks (FT),
upper half of curtain, orographic cloud.
3.-16
result for the upper half of the curtain, with the calculations
started at the 650 mb level, rather than the 700 mb level
as in the case of the lower half. The initial temperature
was -10°C, while over the crest it dropped to -12°C. Particle
concentrations in the fallout run around 100 £-1, and total
precipitation is 63 acre feet with a peak value occurring
over Lake Tahoe. The particle sizes are overall much smaller
than in the Figure 3.1-8 case, and consequently the precipitation
trajectories are flatter, indeed they get started later, and
the precipitation does not begin until about 25 km upwind
of the crest. Thus, the behavior of the upper half of the
curtain is considerably different from the lower half, not
only as a result of the wind shear, but more importantly as
a result of the differing terminal velocities.
Figure 3.1-10 is a horizontal section of the signature
and fallout positions for the two halves. It is seen that
the ground track of fallout particles is shifted considerably
northward from the signature, due to the blocking type flow
characteristic of this PET. In the upper half the distortion
is not so great.
Two upper and lower half instantaneous footprints (FP)
are shown. These truly represent only the upper and lower
half midpoints. Their size is based upon the effects of lateral
dispersion from the initial curtain up through both the signature
and fallout stages. A "total footprint" is outlined by the
wavey line. It includes both of the half footprints over
all times plus some extension to take into account the transport
and dispersion between the very top and the very base of the
curtain. It is seen that this extensive footprint gets to
be around 35 km wide.
3-17
71
E 66
oX
56
LlJ
uz 45
~
I- 34 en
0 24
>- 13
246.5° - 66.5° RADIAL
60 72 84
X DISTANCE (km>
Fig. 3.1-10 Horizontal section of signature (5), precipitation
track on ground (GT) , 3 fallout tracks (FT), and footprirts upper
and lower half midpoints (FP).
An acre-foot of water is 1.234 megatons of water. If
80 acre-feet fell over 2500 km2 (as above), then the J~ecipitation
would average .04 mm on this area. In a typical orographic
cloud the shear and terminal velocity effects spread seeded
water over an extraordinarily large area. If five non··overlapping
curtains could be laid in an hour the total mean pl'ecipitation
would be .20 mm according to the above calculations.
The Convective Case - Basic Concepts~ Convection
of some kind is present in about 85% of the cases sampled
by radar. It takes several different forms and these have
been categorized into the various PET types: In most of these
there is a mesoscale organiza tion •. In the cellula.r types (C1
and C2)' however, there is no such organization, although
they appear most prominently within an elevation range on
the upwind slope. It is these latter types that will be discussed
in the following.
3-18
The GUIDE model as presented so far might be adapted
to growth and fallout from a cell into an orographic cloud,
but something more needs consideration in order to put curtain
seeding within the cell itself into proper perspective. Figure
3.1-11 summarizes the present concept of the kinematics of
cells over the SCPP area. A stern zone contains an updraft
where LWC may approach adiabatic values. Above lies a top
zone where the inflow from the stem zone diverges laterally
and upward. There is a net mass flow into the top from the
stem zone (mean stem updraft Ws ) and outward from the top.
Since the top area exceeds the side area greatly, this flow
is best represented by a mean top updraft Wt which will be
much less than Ws as the top area is much greater than the
stern area. In the configuration shown in Figure 3.1-11 the
P
---.F3
F) =Cr Q9
~- -t--,....------+--'
t Ws
~
TOP -+- STEM
~
Fig. 3.1-11 Hushroom cell concept.
Typical Cell:
Top area =
Stem area =
Depth =
Duration =
100 km2
33 km2
1000 m
2- hour
3-19
whole cloud resembles a mushroom, however there may )e multiple
stems feeding a divided stream of air and liquid water up
into the top.
Simultaneous with the upward flows there is r~pid mixing
of environmental air with the top air (shown by dashed arrows
in the figure) which leads to a net flux of liquid water (F2)
and ice water (F4) out of the top. In addition, there is
the flux of liquid water (Q) from the stem into the top (F1),
and a conversion rate for liquid water into ice water (F3)
as a result of nucleation. New nuclei (capable of forming
ice embryos) are supplied continuously from the stEm, becoming
activated in the lower temperature of the top zone. Any ice
multiplication would be part of the total supply of ice embryos.
There is also a flux of ice water out of the top due to sedi­mentation
(P). The flux of ice F4 upward out of the top,
or downward as precipi ta tion, is proportional to thE! difference
between the mean top updraft W2 and the particle terminal
velocity.
The net mass flow out of the top is controlled by this
same mixing process, which very likely depends upon the air
mass wind shear near the top. With more shear, the mixing
is greater, and the total volume of the top (essentially the
top area) will be less as the mixing depletes the inflow from
the stem more rapidly.
The table in the legend of Figure 3.1-11 shows the typical
dimensions based upon recent plots of cells. Radar cells
for the 1976-77 and 1977-78 seasons combined showed larger
cells but this may have been the result of merging echoes
through a large depth.
3-20
The water balance equation in finite step form is:
#l =
fit
flQl =
fit
Fl - F2
k
MJ1k
L: Nk fit -
Ie
L: N. ~
.ok fit
k
L: (Wt - Vk) NIe r\
where Q is the LWC in the top, Q1 the IWC, Mk is the mass
for the kth particle set, Nk is its concentration, and Vk
its terminal velocity. The flux F2 is proportional to the
top LWC (Q) value, where the constant C2 equals C1, C1 being
the proportion of the top volume filled (or displaced) from
the stem during the time step t. The growth term for ice
particles (the third on the right in the first equation) is
summed over the K ice particle sets. The last term in the
second equation takes care of the losses due to flux of ice
F4 out the top and precipitation out the bottom. This term
has an upper limit equalling the total top ice water content
for a given set k. For steady state conditions the left side
of both equations is zero. A numerical simulation of the
developments from an initial state supports the concept that
a quasi-steady state does develop. The calculations were
based upon a stepwise solution to the equations, starting
with an assumed nucleation level, stem LWC value, and character­istic
dimensions. The GUIDE mass growth equations were used.
The computations were carried out in successive 300 second
steps. After quasi-steady state was achieved the smaller
ice particles, below about 300 m size, or 10- 5 . 5 grams mass,
had concentrations of about 60% of the active nuclei concentra­tion.
These are fairly representative of those measured by
the 2D-C probe. The remaining large ones, corresponding best
to 2D-P particles, had concentrations of around 15% of the
small ones. rhis relationship fits the values observed in
SCPP. It should be noted that in this numerical calculation
3-21
a spectrum of particle sizes develops. The spectrum is not
forced into any pre-conceived distribution. As new particles
are generated in each 300 second step, older ones are lost
either as precipitation out of the base or evaporation out
of the top.
The results with respect to 2 D-C concentrations (arbitrarily
60% of the input nuclei concentration) and quasi-steady state
liquid water content are shown in Figure 3.1-1:!. Each of
the numbered curves is for a different cloud having the typical
dimensions shown in the Figure 3.1-11 legend and having stem
LWC (plus condensation in top) ranging from a high value of
2.5 g m- 3 to a low value of 0.5 g m- 3 • The stem updraft was
2 ms- 1 • The numbers lying along each line are the calculated
steady state precipitation rates in mm hr- 1 •
Looking at the line marked by stem LWC and Top Cond. =
2.5 g m- 3 at its base, we see that if the measured :~ D-C concen­trations
were .06 per liter then the model predi~ted steady
state precipitation rate would be .07 mm hr- 1 ind the LWC
would be .2 g m- 3 • On the other nand, if the mea.sured 2 D-C
concentrations were .6 i-l (due to a colder cloud, or more ice
multiplication, or artificial nuclei), then the steady state
precipitation rate would be .20 mm hr- 1 , the LWC .2 g m- 3 •
At a still higher 2 D-C concentration of 6 i- 1 the precipitation
rate would be .29 mm hr- 1 , the LWC .2 g m- 3 • Now if the 2 D-C
concentrations were 60 i- 1 , then the precipitation would be
reduced to .21 mm hr- 1 , and the LWC to < .02 g m- 3 • A still
higher 2 D-C concentration of 600 i- 1 essentiall~ eliminates
both precipitation and LWC, the steady state cl)ud being a
swarm of small ice crystals.
3-22
3
ZERO
\
2
-u
zou
UIA
N
oLL. 0
C) o
..J
.21
.04
.02
.5
.29
.20
.06 .07 .'- STEM LWC
..!.:.Q. b2.~ + TOP CONDo
<.02 .1 .2.3
WATER (g m-3 )
Fig. 3.1-12 Precipitation (mm hr- l ) vs. 2 D-C concentration
and liquid water content for different clouds.
3-23
Lines associated with other stem LWC values show a similar
trend with larger 2 D-C concentrations. For each line there
is a critical 2 D-C concentration beyond which clverseeding
occurs. Seeding would presumably enhance precipitation only
if it increased the observed 2 D-C concentration j.n this type
of cloud (actually three clouds with different stem LWC values),
but not beyond the critical value.
On an average, it takes over an hour for the quasi-steady
state to develop. The LWC is, of course, initially high and
the IWC low, as might be the case should a new convection
cell suddenly rise from below. Within about 30··40 minutes
the LWC has decreased halfway toward its steady state value
and the IWC has increased halfway toward its steady state
value, and the precipitation now exceeds its steady state
value. Thereafter the steady state value is approached.
The sequence of events moves more rapidly with high nuclei
content, more slowly with less. This suggests that the best
time to seed is in the starting stage (even before a radar
return develops) when the LWC is large.
A Figure 3.1-12 type plot for the 30-45 rrinute stage
of development would show, for a given 2 D-C concentration,
considerably higher ~WC and precipitation values. However,
the same general trend of values with increasin~; 2 D-C con­centrations
would prevail.
This numerical exercise demonstrates the plausibility
of the basic cell concept and the usefulness of locating values
on a 2 D-C by LWC plot (which are observables) in order to
draw inferences concerning seedability. The ac";ual numbers
in Figure 3.1-12 are uncertain due to uncertain inputs. For
3-24
example, if the cloud top depth were twice as great, the precip­itation
would double.
In a given cloud the actual observations do not fall
on a point, but scatter widely. A characteristic envelope
would cover the entire region of the lines in Figure 3.1-12.
The reason for this scatter is that there are many small scale
up and downdrafts (see Figure 3.1-11) within the top containing
different microphysical properties. In real life different
clouds, seeded or not, would have statistically different
ensembles of points.
The analysis assumes dispersion of artificial nuclei
through the top volume that is much more rapid than is possible
through seeding. At a one-sided 1.5 ms- 1 dispersion rate
(typical for convection) it would take 56 minutes to widen
a curtain across this typical cloud, thus consuming much of
the typical cell life remaining after an average start of
seeding. This dictates reliance upon the sort of curtain
dispersion calculations covered above in the stable orographic
cloud case, the main difference being that the cell duration
has to be considered.
It should be pointed out that to predict precipitation
on the ground, it is necessary to add growth that occurs during
the fallout of the particles from the top down through any
orographic cloud present. This growth would probably be
by riming. It is conceivable that heavy cell seeding might
reduce precipitation from the cell base, yet increase it at
ground level due to a greater sweepout of water from the orographic
cloud. But the average orographic cloud for the cell PETS
is relatively shallow.
3-25
Finally, within convection there is always a potential
for dynamic enhancement of the vertical and other ci~rculations
leading to a higher condensation and precipitation rate.
This will be discussed further below.
3.1.4 Some Guide Runs Illustrating ProblE~ms in Cell
Seeding. In order to handle the placement of a curtain into
a convective cell, the procedure applied to the orographic
cloud is modified in the following ways:
1) A charcteristic (or observed). cell mean updraft,
mean LWe, mean top, mean base, and mean duration is input.
The duration may be limited to the time taken for the curtain
to fill the cell, on the assumption that beyond that time
there is no further source of water.
2) The curtain is sUbject to both the mean convective
updraft lifting and the orographic lifting. HowEver, it is
not allowed to move out of the top.
3) The fallout particles remain in the cell environment
only during a duration that is input. Otherwise they fall
into the orographic environment (i.e., the LWe pattern) for
the cellular types. At higher levels this is dry and this
means it would normally evaporate, but a minimlLm particle
size is maintained in order to trace out where the residue
nuclei would move.
Figure 3.1-13 shows the signature and faL.out tracks
for a cell that is curtain seeded a t the 700 mb lE~vel (lower
half) at the zero x coordinate. The mean LWe was 0.35 g m- 3 •
The e2 mean sounding was used. It was assumed that tne cloud
depth was 100 mb, and coincided with the curtain. Fallout
3-26
500
.c 600 -13.8°
E
w
(z.) 700
.<..l.:
en
0 800
N
900
1000
12 24 36 48 60 72 84 96 lOB 120
X DISTANCE (km)
Fig. 3.1-13 Signature and fallout tracks, lower half of
curtain, cell cloud.
tracks FT1 and FT2 bound all the fallout from the cell. The
lower half curtain produced about 280 acre feet of water.
Fallout tracks FT3 and FT4 bound all the fallout originating
outside of the cell (or after the curtain has filled it).
The duration was taken to be only 2400 seconds in order better
to illustrate the change in trajectories. Under normal conditions
the curtain would be within the cell and effective twice this
long.
Figure 3.1-14 shows the pattern for the upper half of
the curtain. Because of the colder temperature and therefore
longer trajectories, the precipitation reaches the ground
only on the upper portion of the barrier.
3-27
400
500
....
D 600
E
IlJ
U 700
z
<t
I-en
0 800
N
900
1000
0
X DISTANCE (km)
Fig. 3.1-14 Signature and fallout tracks, upper half of
curtain, cell cloud.
In the process of rapidly converting an ever-enlarging
volume of liquid water into ice, the curtain is releasing
additional heat, which in a cloud having a near wet adiabatic
lapse rate would lead to the development of new eonvection.
Such an artificially induced convective circulation would
most likely take the form of an upward motion in t.J.e heart of
the curtain, with divergence aloft, and subsiding motion outside
of the curtain. The process is illustrated in Fig'lre 3.1-15.
Additional condensation through the 1000 m curtain depth would
provide a moisture supply rich in comparison to o")served LWC
values, and thus lead to considerable enhancement of precipi­tation.
A dynamic result of curtain type seeding could thus
produce a small scale convection band. This co~sideration
3-28
/ / /!,'
CLOUD
I ./ / :! / / / / / / I
I ! / / 1.1 I I III
/ / ! / I / J' / / /
i / .I / / / ! /' / / / ///
/' i I / ;
! ! " / I !
I / !
/
/
t t CURTAIN
I
I
/ .I
/ / I I / /
) i / I / /
/ ,I / I /
/ I I
/ /.
/ / /
/ I
CLOUD
I
,I
/
/
I
/
/ / , .
Fig. 3.1-15 Curtain dynamic effect.
calls for detailed aerial and radar sampling of the curtain
itself as the unit of observation.
Some Water Balance Considerations. The foregoing
provides the conceptual basis for a GUIDE model that can be
used both to guide seeding operations and to aid in the evaluation
of results. Only the application to the orographic and cellular
PET's using the aerial curtain type seeding mode has been
addressed in this discussion. Adaptations to other PET's
and modes is underway.
3-29
GUIDE focuses on a Lagrangian type tracing of fallout
from a signature, with diffusion (dispersion) effects· included
in a simplified manner. GUIDE does not compute the full water
balance and seeding produced changes therein. lt assumes
that there is a pre-existent natural state wherein ~ specified
distribution of LWC exists (and is observed) that can serve
as a reservoir of LW that artificial nuclei can convert to
ice particles, thus capturing some LW that might otherwise
either be lost in lee-side evaporation or in cloud top evapor­ation.
The examples read out acre feet of added precipitation,
but these figures do not take into account what the interaction
is with the natural water balance. The volume (:overed by
the curtain and its fallout onto a footprint is certainly
subject to a sUbstantial, but not necessarily total, conversion
of any available LWC to ice particles, usually far exceeding
any natural conversion. These particles will very likely
fallout downwind of where natural particles would have fallen.
At some distance downwind from the heavily seed(~d initial
curtain any natural particles present would be deprived of
LWC upon which to grow. First of all, it seems likely that
there is a net downwind transfer of precipitation from lower
to higher elevations in the ARB. This transfer w(luld extend
on over into the Tahoe Basin. Secondly, by reducing evaporation,
there could be a net increase of precipitation over the entire
barrier.
3.2 Task Force 9 - Comparative Experimentation: Some Principles
and Prescriptions
The careful conception, design, implementation, analysis
and reporting of any experiment is not an easy task. A statement
by Benjamin Franklin succinctly summarizes some of the problems
that every experimenter encounters on the usually rougb Journey
3-30
from the conception of an experiment to its final reporting.
"While my care was employ'd in guarding against one fault,
I was often surprised by another; habit took advantage of
inattention; inclination was sometimes too strong for reason."
The Autobiography of Benjamin Franklin, 1774. Furthermore,
when the experiment is completed the experimenter desires
to announce to the world his results and often to recommend
an action. As such, the experiment and its results should
have substantial "believability" and must withstand scrutiny
and further investigation.
To produce an experiment whose results are "believable"
does not involve running, typically at the last moment, to
a consulting statistician for appropriate anointment or sanctity.
Rather, the achieving of this "believability" status or class
should be based on the statistical credibility of the full
experiment and the scientific plausibility of the offered
conceptual model. Thus, the basis for an experiment's "believ­ability"
must be laid early in its lifetime (preferably at
the onset of the discussion of the experiment) and maintained
by proper attention to the numerous components, states, and
details that constitute a "believable" (or proper) experiment.
In Section 3.2.1 a brief discussion of believability
and how it pertains to scientific experimentation is given.
Section 3.2.2 presents the general anatomy of a be

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INTERIM REPORT NO. 4
CONTINUING DESIGN WORK FOR THE
SIERRA COOPERATIVE PILOT PROJECT
REPORT NO. SLWM-81-3
PREPARED FOR
THE UNITED STATES DEPARTMENT OF INTERIOR
BUREAU OF RECLAMATION
OFFICE_OF ATMOSPHERIC RESOURCES RESEARCH
CONTRACT NUMBER 7-07-83-V0008
BY
R.D. ELLIOTT AND D.A4 GRIFFITH
NORTH M1ERICAN ~~EATHER CONSULTANTS
1141 EAST 3900 SOUTH, SUITE A130
SALT LAKE CITY, UTAH 84117
AND
JOHN A. FLUECK
FLUECK ASSOCIATES
WYNCOTE, PENNSYLVANIA
. AND
JACK A. HANNAFORD
SIERRA HYDROTECH
PLACERVILLE, CALIFORNIA
SEPTEMBER 1981
WS-2S0 (2.72)
Bure~u of Recl.&m&tloZl
1. REPORT NO.
4. TlTl.E ANO SUBTITl.E
S. REPORT OATE
North American 1~eather Consultants
1141 East 3900 South, Suite A-130 11. CONTRACT OR GRANT NO.
Salt Lake City, Utah 84117 7-07-83-V0008
I-;;r:-SP'Qi:;iSCiiRiNC-;:GiN;:::yi:iA-.:ii;--r;:;r;-r;:;;:;:;;;:='«----------~ 13. T Y P E 0 F REP0 RTAN0 PERI 0 0
112. SPONSORING AGENCY NAME ANO AOORESS COVEREO
INrERD1 REPORT ~K). 4
CONTINUING DESIGN WORK FOR THE SIERRA
COOPERATIVE PIWf PROJECT
7. AUTHOR!S)
R. D. Elliott, D. A. Griffith, J. A. Flueck,
and J. F. Harmaford
5. PERFORMING ORGANIZATION NAME 1.1010 AOORESS
Office of Atmospheric Resources Research
Bureau of Reclamation
ro Box 25007
Denver Colorado 80225
15. SUPPLEMENTARY NOTES
Septer.1ber 1981
6. PERFORMING ORGANIZATION COOE
8. PERFORMING ORGANIZATION
REPORT NO.
10. WORK UNIT NO.
Interim
a. SPONSORING AGENCY COOE
16. ABSTRACT
Research activities ot North Amprican Wpather Consultants (~AWC) and two sub~untrac­tors,
flueck Associates and Sierra Hydrotech, during the period October lY~O through
September 1981 are described. This research was concerned with continuing design
work on the Sierra Cooperative Pilot Project (SCPP) being sponsorpd oy the Burpau
of Hpclamation in the northern Sierra Nevada of California and ~evada.
Seeding scenarios are presented for aerial curtain seeding of major precipitation
bands and tor ground based seeding of post-frontal convective cells. It has bpen
determined that major bands commonly occur in the SCPP target area. The seeding
scenarios dpal with recommended treatment modes as well as estimates of potential
cloud seeding effects.
Two reports are presented which summarize work performed by NAwC and its subcontractors
as part of the detailed analysis performed on the SCPP during the 19~O-81 time
frame. These reports are concerned with dispersion of sepding material in the
SCPP and statistical design considerations of an exploratory phase of the SCPP.
NAwC and its SUbcontractors prepared five ditferent documents dealing with various
aspects ot the design of an exploratory phase on the SCPP. These documents discuss
the following topics: 1) treatment design, 2) a seeding guidance computerized
model, 3) seeding 'suspension criteria, 4) extra area effects of seeding, and 5)
statistical evaluations.
Two analyses are presented that were completed during the year dealing with estimation
of the required precipitation gage density and locations and the relationships
between various air mass characteristics versus time before or after upper-level
trough or frontal passage.
17. KEY WOROS ANO OOCUMENT ANAL.YSIS
a. OESeR IPTORS--
1~eather modification
Cloud seeding
Artificial nucleation
b. IDENTIFIERS-- /Project Skywater/Sierra. Cooperative. Pilot Project/Sierra
Nevada
c. COSATI Field/Group
18. OISi~l'i!l,\)~'t)~STATEMENT
Avoilable from the National Technical Information Service. Operations
Division, Sprin,fjel6, Virrinio 221SI.
NA
15. SECURITY C LASS Ill. NO, OF PAGE~
(THIS RCPORT)
U-N'CLASS IF lED
20. SECuRITY Cl.ASS 22. PRICE
(THIS 'Alitl
UNC LASSIF lED
TABLE OF CONTENTS
Section
ABSTRACT •••••••••••••••••••••••••••••••••••••••• i
1. INTRODUCTION .............•...........••......•.. 1-1
2. SCENARIOS FOR CURTAIN SEEDED MAJOR BANDS AND
G~OUND SEEDED CELLS •••••.••••.•••••••.••.•••..•• 2-1
2.1 Background and Conceptual Models ••.•.....•. 2-1
Hypotheses and Response Variates 2-43
2.3 Seeding Modes and Procedures ••••••••••....• 2-48
2.4 Estimate of Natural and Augmented
Precipitation and Frequency of Occurence ..• 2-60
2.5 Gaps in Knowledge ••••.••••••••••••••.•••••• 2-03
3.1 Task Force·6 - Transport and Diffusion •••••
3.2 Task Force 9 - Comparative Experimentation:
Some Principles and Prescriptions •••..••
3 • TASK FORCES 6 AND 9 ............................. 3-1
3-2
3-30
4. SCPP-1 DRAFT DESIGN APPENDICES .•..••.•••..••.... 4-1
4.1 Appendix D: Treatment Design ••.•••......•• 4-1
4.2 Appendix H: The Seeding Guidance Model ••.• 4-13
4.3 Appendix I: Suspension Criteria .•.•.•..•.• 4-35
4.4 Appendix J: Extra Area Effects ..••.••..•.. 4-58
4.5 Appendix N: Statistical Evaluation •••...•• 4-87
5. MISCELLANEOUS ANALySES.......................... 5-1
5.1 The Designing of a Precipitation Gage
Network for the SCPP •••••.••••••.••.••••.•• 5-1
5.2 SCPP and CRBPP Rawinsonde-Derived Parameters
versus Trough or Frontal Passages •••.•••.•• 5-13
REFERENCES •..••.••••.••••..•••...•••••..•.•....•• 5-17
i
List of Figures
2.1-1
2.1-2
2.1-3
2.1-4
2.1-5
2.1-6
2.1-7
Typical LWC and ICC distribution in
orographic convection ••.•..•.•••••• , ••,••••
Schematic of convection bubble model ••••. ••••••
Schematic of curtain being entrained
into bubbles , .
Seeding effects in parameter space •••••• , ••.•••
Distribution of particle mass ••••.•••••• , ••••••
TB-1 AgI curtain-orographic, curtain 30 krrl
downwind and 300 mb above valley floor.
Sounding type area wide-orographic .
TB-1 AgI curtain-orographic, curtain 30 kIn
downwind and 350 mb above valley floor.
Sounding type area wide-orographic •.••••••
2-8
2-10
2-11
2-13
2-18
2-20
2-21
2.1-8
2.1-9
2.1-10
2.1-11
2.1-12
2.1-13
2.1-14
2.1-15
2.1-16
TB-1 AgI curtain-orographic, curtain 30 kill
downwind and 400 mb above valley floor.
Sounding type area wide-orographic •••••••• 2-22
TB-1 AgI curtain-orographic, curtain 30 kn
downwind and 450 mb above valley flo~r.
Sounding type area wide-orographic ••.••••• 2-23
TB-1 AgI curtain-orographic, curtain 30 km
downwind and 500 mb above valley floor.
Sounding type area wide-orographic .•••..•• 2-24
TB-1 AgI curtain-orographic, curtain 0 km
downwind and 400 mb above valley floor 2-25
Aerosystems AgI curtain-orographic, curtain
30 km downwind and 350 mb above valley floor
Sounding type area wide-orographic •.••••••. 2-26
NAWC AgI curtain-orographic, curtain 30 km
downwind and 350 mb above valley floor.
Sounding type area wide-orographic •.••••••• 2-27
Nuclei production curve versus
temperature, TB-1 •••••••••••••••••.•.•••••• 2-29
Nuclei production curve versus temperature
for TB-l and Aerosystems silver iodide-acetone
generator •••••.••••••••••...•.•.••• 2-30
Nuclei production curve versus temperature
for TB-l and NAWC's silver iodide - acetone
generator , 2-31
11
List of Figures
2.1-17
2.1-18
2.1-19
2.1-20
2.1-21
2.1-22
2.1-23
2.1-24
2.1-25
2.3-1
2.3-2
AgI curtain-convection, curtain 15 km
downwind and 300 mb above valley floor,
top of convective instability at 350 mb
above valley floor. Sounding type -
major band 2-33
AgI curtain-convection, curtain 15 km
downwind and 300 mb above valley floor,
top of convective instability at 400 mb
above valley floor. Sounding type -
major band 2-34
AgI curtain-convection, curtain 15 km
downwind and 300 mb above valley floor,
top of convective instability at 450 mb
above valley floor. Sounding type -
major band 2-35
AgI curtain-convection, curtain 15 km
downwind and 300 mb above valley floor,
top of convective instability at 500 mb
above valley floor. Sounding type -
major band 2-36
C02 curtain-convection, curtain 15 km
downwind and 300 mb above valley floor,
top of convective instability 400 mb
above valley floor. Sounding type -
major band •••••.••••••••••••••••••.•...•••• 2-37
AgI point source-convection, ground source
20 km downwind and 40 mb above valley floor,
top of convective instability 350 mb above
valley floor. Sounding type - Ct ••••...•••• 2-38
AgI point source-convection, ground source
20 km downwind and 40 mb above valley floor,
top of convective instability 400 mb above
valley floor. Sounding type - C1 ••••••....• 2-39
AgI point source-convection, ground source
20 km downwind and 40 mb above valley floor,
top of convective instability 450 m above
valley floor. Sounding type - C1 ••••.•••••• 2-40
AgI point source-convection, xy plot.
Sounding type C1 •••••.••••••••••.••••.••.•• 2-42
Convective band schema tic •.••.•••••••••••.••... 2-49
Mean sounding for the 1980 forecaster
designated major band echo (PET) •••••••.•.• 2-51
iii
List of Figures
2.3-3
2.3-4
2.3-5
2.3-6
2.3-7
Possible seeding pattern in a major band •.•••.• 2-54
Mean vertical profiles of equivalent
potential temperature (solid lines)
displayed with low level maximum 9 e
isotherms (dashed lines) by forecaster
designated PET in 1980 •••.••••••••.••••••. 2-57
Mean sounding for the 1980 forecaster
designated cellular echo type (C1) •••..••• 2-58
Mean sounding for the 1980 forecaster
designated cellular echo type (C2) •••..•.. 2-59
Possible remote controlled ground generator
network 2-61
3.1-1
3.1-2
3.1-3
3.1-4
3.1-5
3.1-6
3.1-7
3.1-8
3.1-9
3.1-10
3.1-11
3.1-12
Orographic cloud concepts •.•.•••••••.••.••.••••
Precipitation change in fallout columns
with seeding (mm hr- 1 ) ••.•..•.......••..••
Typical stable flow pattern ••.•••.•••••••••••••
Typical V component ••••••••••.•••••••••••••••..
Typical flare expansion (circles) and
center dispersion (arrows) ••••••..••••••..
Typical curtain spread .••••••••.••.••••.••••.•.
Schematic of signature and fallout steps ••••...
Signature(s), 5 fallout tracks (FT), and
one fallout plume (FP), lower half of
curtain, orographic cloud •.•.•••.........•
Signature(s), and 5 fallout tracks (FT),
upper half of curtain, orographic cloud •.•
Horizontal section of signature(s),
precipitation track on ground (GT), 3 fallout
tracks (FT), and footprints upper and lower
half midpoints (FP) ••••••••••••.••..••.•..
Mushroom cell concept •• ~ •••.•.•••••••••. .•.•...
Precipitation (mm hr- 1 ) vs 2 D-C concentration
and liquid water content for different
3-3
3-4
3-5
3-6
3-8
3-9
3-12
3-15
3-16
3-18
3-19
3.1-13
3.1-14
c lauds 3-23
Signature and fallout tracks, lower half
of curtain, cell cloud ••••••••••••..•••••. 3-27
Signature and fallout tracks, upper half
of curtain, cell cloud 3-28
iv
List of Figures
3-37
3.1-15
3.2-1
Curtain dynamic effect .•••.••.•••••••••.••..... 3-29
The flow diagram of the contrast between
the "Investigator" and the "Real Word" ••••
4.1-1
4.2-1
4.2-2
4.2-3
4.2-4
4.2-5
4.2-6
4.2-7
Map of the proposed SCPP-1 target area with
a 10 km precipitation gage spacing. Gages
adj usted to 1-80 and US 50 ••••••.•••.•••••
Airflow normal to barrier ••••••.••••••••••••.••
Model predicted trajectories ••••.•••••.....•..•
Horizontal section •............................
Cell cloud and curtain at 2 times ••••.•••••••••
Typical cell history ••••.•••••••••••....••••.•.
Plot of cell runs .
Profiles of liquid water and ice along the
barrier (by step) for the case with no
4-2
4-15
4-15
4-16
4-17
4-18
4-19
4-22
4.2-8
4.2-9
updraft 4-21
Profiles of natural precipitation along
the barrier for the case with no updraft
Profiles of liquid water and ice content
along the barrier (by step) for the case
with an updraft •.•..••••••••••••.....•..•. 4-23
4.2-10
4.2-11
4.2-12
4.2-13
4.2-14
4.2-15
4.3-1
Profiles of natural precipitation along
the barrier for the case with an updraft ••
Seeded precipitation profiles for seeding
the case with no updraft with a C02 curtain
Seeded precipitation profiles for seeding
with C02 curtain, with an updraft •••••••••
Warm cell (with updraft) seeded precip-itation
for C02 curtain seeding at two
locations, and natural precipitation .•.•.•
Moderate cell (with updraft) precipitation
for C02 curtain seeding at two locations,
and natural precipitation •••••••••••••.•.•
Matrices of log of particle concentration
(NI m- 3 ) and log of particle mass (MI g) by
step (row) and by set (column) for a warm
natural cell case having no updraft ••....•
SCPP .experimental area ••••.••••••••••••.•..••.•
v
4-23
4-25
4-26
4-27
4-28
4-30
4-37
List of Figures
2.3-3
2.3-4
2.3-5
2.3-6
2.3-7
Possible seeding pattern in a major band ...•••• 2-54
Mean vertical profiles of equivalent
potential temperature (solid lines)
displayed with low level maximum e
isotherms (dashed lines) by forecaster
designated PET in 1980 •••••.•.••....•.••.• 2-57
Mean sounding for the 1980 forecaster
designated cellular echo type (C1) •••••..• 2-58
Mean sounding for the 1980 forecaster
designated cellular echo type (C2) •.••.••• 2-59
Possible remote controlled ground generator
network 2-61
3.1-1
3.1-2
3.1-3
3.1-4
3.1-5
3.1-6
3.1-7
3.1-8
3.1-9
3.1-10
3.1-11
3.1-12
Orographic cloud concepts •••..••.•.••••..••..••
Precipitation change in fallout columns
with seeding (mm hr- 1 ) ••.•.••••......••••.
Typical stable flow pattern •.•••••.•.•••..•.•.•
Typical V component •.•••.••••••••••••..••..•••.
Typical flare expansion (circles) and
center dispersion (arrows) •••••••.•..•••.•
Typical curtain spread •...••.....••..••.•••••••
Schematic of signature and fallout steps .•••.••
Signature(s), 5 fallout tracks (FT), and
one fallout plume (FP), lower half of
curtain, orographic cloud •..•••..••...••.•
Signature(s), and 5 fallout tracks (FT),
upper half of curtain, orographic cloud •••
Horizontal section of signature(s),
precipitation track on ground (GT), 3 fallout
tracks (FT), and footprints upper and lower
half midpoints (FP) •••••••...••...•••.•••.
Mushroom cell concept ••••.•••.•...••.•••.•••.•.
Precipitation (mm hr- 1 ) vs 2 D-C concentration
and liquid water content for different
3-3
3-4
3-5
3-6
3-8
3-9
3-12
3-15
3-16
3-18
3-19
3.1-13
3.1-14
clouds 3-23
Signature and fallout tracks, lower half
of curtain, cell cloud •••••••••••••....••• 3-27
Signature and fallout tracks, upper half
of curtain, cell cloud 3-28
vi
List of Figures
4.3-2
4.3-3
Historic flood conditions. Smith Fork
American River near Lotus, December, 1955. 4-41
Historic flood conditions. South Fork
American River near Lotus, January -
February, 1963 ..•......................... 4-42
4.3-4
4.4-1
4.4-2
4.4-3
4.4-4
4.4-5
4.4-6
Historic flood conditions. South Fork
American River near Lotus, December, 1964 •
SCPP primary experimental area ••.••.••••••...••
Sierra Cooperative Pilot Project primary
study area plus extended areas 1 and 2 ••.•
NWS precipitation gage network ••.••.•.••••.•.••
197~-80 SCpp precipitation gage network ••••...•
197~-80 SCPP meteorologicval network •.•...•..••
1979-80 cooperator's precipitation gage
4-43
4-64
4-65
4-66
4-68
4-70
4.4-7
4.4-8
4.4-9
4.5-1
4.5-2
5.2-1
5.2-2
5.2-3
5.2-4
network 4-71
Stations reporting hourly weather ••••.•..•••.•• 4-74
Rawinsonde sites ..........................•.... 4-75
Ground generators for SMUD, PG&E, and DRI ••
Schematic of the precipitation chain ••.•..••.•. 4-93
Probable location of the 95% confidence
region for two response variables •....•••. 4-95
Ice cloud, sounding cloud top, and water
cloud top for the SCPP 1976-80 versus
700 mb trough passage time ••••.•.....•••.• 5-15
Ice cloud top, sounding cloud top, and
water cloud top for the CRBPP versus
frontal passage time •.•••.•••.•....••.•••. 5-15
Combined SCPP PETS percent frequency versus
700 mb trough passagetimes ••••••.••.•..•.. 5-16
Mean seeded and not seeded precipitation on
the CaBPP for three groupings of precip-itation
gages versus frontal passage times 5-16
vii
List of Tables
Estimated total sample size for treatment
effects on a proportion (P) wi th a = .05
and S = .20 4-12
The performance matrix for the states and
components of a proper comparative
experiment 3-42
Primary response variables ••••••••••••.•.•••••• 4-11
2.1-1
2.1-2
2.1-3
2.1-4
2.1-5
2.1-6
2.2-1
2.2-2
2.2-3
3.2-1
4.1-1
4.1-2
4.1-3
Natural cloud top nucleation •••.••••.•••.•••.••
Orographic steady state water balance
( stable) .
Natural cloud top nucleation ••••••••••.••..••••
Orographic steady state water balance
(convective) .
Stable orographic summary •••••••••••••.•••••.••
Summary of unstable grid runs ••••••••••.••.••••
Links in seeding chain ••••••••••••.•••••.••••••
Major bands, modified curtain seeding ••••••...•
Cells, ground generator seeding ••••....•....•••
Estimated total sample sizes for ground-level
precipitation as a function of
treatment effect and power. The data. are
from 5 hour periods under C1/C2/ NE
conditions, 1978-80. The a level is
Page
2-3
2-4
2-6
2-7
2-28
2-41
2-44
2-46
2-47
• 0 5 and R = 0 ••• •••• ;..................... 4-12
4.3-1
4.4-1
4.4-2
4.4-3
4.5-1
4.5-2
4.5-3
Recommended cutoff for snowpack accumulatjon
at 180% of average for given date eXIlressed
as percentage of average April 1 watE~r
content for the American River Basin ••.•..
Physical - chemical evidence of Extra
Area Effects •...•.••••.•.......•••. I ••••••
NWS precipi tation gage densi ty ••••••••.• , •••.•.
NWS, SCPP and cooperator gages anticipated
to be in place for the 1981-82 season •.••.
Primary response variables •.••...•..•..• ,' .....•
Secondary response variables •.•.•.•••.•..' •..•••
Parameters to be estimated in SCPP-1 and
their suggested relation ••••••••••. , •.••.•
viii
4-49
4-60
4-67
4-69
4-89
4-90
4-91
List of Tables
5.1-1
5.1-2
5.1-3
General inputs to a network design ••..•....••••
Some empirical interception for various
precipitation gage network grid con­figurations
and spacings •••••..•••••.•..••
The frequency distributions of gage inter­ceptions
for various square grid spacing
Page
5-3
5-9
5.1-4
designs 5-11
The elapsed time of the cells in the
target area 5-12
APPENDICES
Appendix A -
Appendix B -
Appendix C -
Appendix D -
Some Theory of Grid Spacing
Network Grid Designs
The Precipitatin Data for Five Days of SCPP
in 1978-79 and 1979-80 Seasons
The Frequency Tables and Summary Statistics
for the Precipitation Print Variables
ix
Partial List of Abbreviations and Acronyms
AgI
AI
Bureau -
CRBPP -
DRI
ETI
ICC
IWC
LWC
MBA
NAWC -
NE
NWS
PET
PG1E ­SCPP
­SCPP-
l -
SMUD ­UW
silver iodide, a common weather modification
seeding material
Atmospheric Inc., Fresno, California
Bureau of Reclamation
Convective precipitation echo type (PET) with
less than 50% radar coverage over the American
River Basin
same as above except coverage greater than
50%
dry ice, a common wea ther modifica t:_on seeding
material
Colorado River Basin Pilot Project, a Bureau
weather modification research projec·c conducted
in the San Juan Mountains of Colorado
Deseret Research Institute, Reno, Nevada
Electronic Techniques, Inc., Ft. Colli~s, Colorado
and Auburn, California
ice crystal content
ice water content
liquid water content
MB Associates, Palo Alto, California
North American Weather Consultants, Salt Lake
City, Utah and Santa Barbara, California
no echo precipitation echo type (PET)
National Weather Service
precipitation echo type, a cloud classification
scheme developed for the SCPP
Pacific Gas and Electric Company
Sierra Cooperative Pilot Project
a planned first phase of an exploratory research
component of the SCPP
Sacramento Municipal Utility Distric;
University of Wyoming
x
1. INTRODUCTION
This report covers the activities of North American Weather
Consultants (NAWC) and its two subcontractors - Flueck Associates
and Sierra Hydrotech during FY 1981. The FY 1981 activities
on the Sierra Cooperative Pilot Project (SCPP) were different
in a number of regards from several of the previous years
in which the SCPP has been active.
Perhaps the most significant change resulted from a decision
by the Bureau that specified that FY 1981 would be an analysis
year instead of an active field data collection year. The
reason for this was the large amount of field data that had
already been acquired in prior years that had not been analyzed
in detail. This decision coupled with the desire to test
the feasibility of conducting one phase of an exploratory
program on the SCPP set in motion several activities.
The first action that was taken, based upon the decision
to perform analysis during FY 1981, was the organization of
nine separate task forces to address specific questions in
the analysis work. SCPP scientists, both Bureau as well as
contractor employees, were assigned to one or more of these
task forces. A lead scientist was appointed for each task
force. In NAWC's work Robert Elliott and John Flueck were
,
selected lead scientists on task forces 6 and 9, respectively.
Results of the work by the various task forces was reported
on at an analysis conference in Denver May 19-20, 1981. Presenta­tions
by Robert Elliott and John Flueck are contained in Section
3 of this report.
During the performance of the data analysis and out of
a need for adequate lead time to design and set up an exploratory
1-1
program for FY 1982, if such a decision to proceed was reached
in FY 1981, a parallel effort was initiated to construct a
number of possible seeding scenarios. These scenarios were
compiled by various Bureau and contractor personnel and presented
at two different meetings - preliminary scenarios at a special
meeting held in Salt Lake City on January 14-15 I 1981 and
at a design and task force workshop meeting conducted in Auburn
on February 10-12, 1981. Seeding scenarios were developed
for several different PETS (Precipitation Echo Types). NAWC
(Robert Elliott and Don Griffith) prepared one scenario concerned
with seeding major bands with aircraft and convection with
ground based sources. This scenario is contained in Section 2.
Shortly following the formal presentations of the task
force groups at the May conference in Denver, a tentative
decision was made to proceed with the design ot one phase
of an exploratory program for the SCPP starting in o:he 1981-82
winter season, designated SCPP-1. Consequently, work began
on the preparation of a draft design for SCPP-1. Various
Bureau and contractor employees then were requested to prepare
design appendices on a variety of sUbjects to pro'Tide backup
for a design prepared by Larry Vardiman of the Bureau. NAWC
and its subcontractors participated in the preparation of
five such appendices as follows: Appendix D - Trea:ment Design;
Flueck Associates - John Flueck; Appendix H - The Seeding
Guidance Model, NAWC - H.obert Elliott; Appendix I .. Suspension
Criteria, NAWC - Don Griffith and Sierra Hydrotech - Jack
Hannaford; Appendix J- Extra Area Effects, NAWC - ~n Griffith;
and Appendix N - Statistical Evaluation - Flueck Associates ­John
Flueck. These appendices were discussed at a draft design
workshop held in Auburn on July 14-15, 1981. The fi VI:! appendices
are presented in Section 4.
1-2
Some other analyses and developmental work was performed
during the reporting period. Notable among these was the
considerable revision to Robert Elliott's seeding guidance
model. Output and discussion of the model appear in Sections 2,
3.1 and 4.2. John Flueck of Flueck Associates also conducted
a study of the possible desired precipitation gage network
needed to detect precipitation from postfrontal convective
cells (C1 and C2 PETS) during SCPP-1. This work is summarized
in Section 5 and Appendices A-D.
1-3
2. SCENARIOS FOR CURTAIN SEEDED MAJOR BANDS AND GROUND SEEDED
CELLS
2.1 Background and Conceptual Models
The data available for establishing scenarios for full
scale exploratory experiments con~ist primarily of the Sheridan
soundings, air and ground microphysics, radar, and surface
network data. At present only advance and partial analyses
are available so that whatever is designed at this point would
undergo some, but not necessarily radical, change before possible
implementation in the coming winter season.
Observations are for the most part made on a regular
schedule during storms and cover a large portion of the experi­mental
area. However, the cloud microphysics by necessity
is confined to a very small volume swept out by a high speed
aerial platform. The most extensive sample of data for convective
systems is that for cells, while the most limited sample is
that for the major band, the most complex system. The latter
were sampled primarily during January and February 1980, a
period of time when storms came in a series of jet stream
(EJ) weather type, notable for the very tropical maritime
character of its air mass.
In what follows the results of various analyses, mostly
preliminary in nature, will be invoked without giving any
details since the details have already been presented at various
meetings.
Because the seeding process commences by altering cloud
microphysics, it is obligatory first to consider natural micro­physics
for the convective case.
2-1
2.1.1 The Orographic Scale. Scatter plots of super-cooled
liquid water content (LWC) versus ice crystal concentration
(ICC) indicate supercooled liquid water to be most prevalent
in the 0 to _5°C zone, next most prevalent in the _5° to -10°C
zone, and least above. An analysis of the top of the water
saturated cloud as given by the Sheridan sounding shows a
similar frequency distribution. It can be expected that water
saturated cloud would be associated with liquid cloud droplets.
In addition, comparisons of near simultaneous Sheridan and
Freshpond soundings show that the water saturated cloud thickens
on ascent. This observation, along with observations of frequent
riming at ground level high on the barrier, suggest considerable
supercooled liquid water accumulation, in general, at levels
lower than those sampled by the aircraft.
The accumulation of LWC at lower levels up the barrier
is simulated quite well by means of a simple orographic wa ter
balance model (OROGWATER). Some calculations will be discussed.
Table 2.1-1 shows the steady state distribution of LWC
calculated for a cloud with a top 5000 m above va.lley floor
and typical orographic sounding parameters. Na"tural cloud
top nucleation occurs at the top of each of the sev{~n "fallout"
columns, which are literally slanted to the right as one descends
the column. LWC does accumulate at low levels as Ole proceeds
up the barrier, reaching a peak of .5 gm- 3 near the crest.
A simple simulation of seeding is to multiply the natural
nucleation by a factor greater than 1. When this is done
the new steady state distribution of LWC and the new ground
level precipitation appears. Table 2.1-2 shows the results
of several different seeding schedules. The first schedule
multiplies natural nucleation by 5 for 4 columns (each 13 km
2-2
Table 2.1-1. Natural cloud top nucleation.
Distribution of LWC (gm-3)
0.010 0.002 0.003 0.013 0.031 0.052 O.OTI
0.020 0.007 0.007 0.019 0.045 j). Q.S2_ / /6':'141
0.030 0.007 0.020 0.038 0.070 / 0.109 0.179
0.040 0.017 0.023 0.054 .Q .(L~3 ..... ..- 0.132 0.199
0.050 0.019 0.028 0.065 , ... 0.104 0.138 0.194
0.040 0.022 0.057 0.021 ..- 0.102 0.128 0.175
0.030 0.015 0.068' 0.034 I 0.180 (0:271 / /" I" O. i33
0.020 0.010 -0.-050- __0..D'Z..4 .... o 158 0.090 0.128 0.010 0.005
10- 5g) fall to the ground out of the outflow region (PWC).
In the case of emergent convection they fall a considerable
distance in dry air, but are too large to completely evaporate
before reaching any lower orographic cloud, where they may
grow further by accretion.
In this simulation new particle sets (naturally or arti­ficially
nucleated) are produced at each step, in accordance
with the step mean temperature. In the natural case the standard
2-9
HEIGHT IE ;) DISTANCE
POSITION NO.
STEP NO.
4
3
2
-I
INFLOW
-3
-5
OUTFLOW
\PWC
~ LViC
~IWC
Fig. 2.1-2 Schematic of convection bubble model.
2-10
exponential background formula is used, but ice multiplication
between the -40 and -80 C levels can also be introduced. With
artificial nucleation a curtain that has been in existence
long enough to produce a reasonable (and computable) average
ice nuclei concentration is introduced near the base of convec­tion.
A curtain of less thickness than the 1 km standard
used in the calibration runs would be preferable; i.e., its
depth should be comparable to the inflow region depth. The
curtain is assumed to have expanded to where its area is sufficient
to encompass one or more newly developing bubbles. This may
require an hour. Figure 2.1-3 depicts the process.
CURTAIN
Fig. 2.1-3 Schematic of curtain being entrained into bubbles.
2-11
The nucleation and growth of each set of paJ~ticles is
computed stepwise. There may be one set nucleated in step
one, another in step two, etc. If there are 10 steps to the
outflow region, then there will be 10 sets of particles there.
The particle spectrum is thereafter divided into the three
aforementioned categories and the water content of each calculated
(PWC, IWC and LWC). The crucial one is, of course, the precipi­tation
water content (PWC).
An important part of BUBBLE is the entrainment factor,
a factor of obvious importance in the Sierra where the total
water content (LWC and IWC) in convection is observed to be
much lower than the adiabatic value. The momentum of the.
environmental wind (2/3 cellular mean sounding) is entrained
and mixed at each step with that advected up from below so
that the horizontal motion of the bubble can be char":ed.
Computations were made to compare seeded and natural
production of PWC at the outflow region position (and at all
lower positions) for a variety of updraft rates and cloud
depths. The convection base was set at the 850 mb level.
Figure 2.1-4 shows the results wi th respect to area.:; favorable
or unfavorable for positive seeding effects on a cloud depth
versus updraft plot. Total bubble duration lines are also
shown. These lines slope because duration depends upon both
updraft and depth. The duration includes residence time in
an outflow region that extends outward to where the prl~cipitat ion
particles have fallen out of that region. This adds about
1000 sec to the whole duration. It is estimated fron presently
available SCPP climatology that the vertical scale embraces
about 80% of convection cases and the horizontal scale about
the same for updrafts.
2-12
3000 5000
POSTIVE Eo(-­EFF
CT
2000
3500 -19
4500 -24.5
4 1.6
UPDRAFT (ms-I)
1.0
Fig. 2.1-4 Seeding effects in parameter space.
2-13
Figure 2.1-4 indicates that in about half of the total
area positive effects are expected. Such ef fec ts are s,ubstan tial
(several fold), whereas in the rest of the area, seed-natural
differences are small and negative.
The effect of dynamically produced enhancement of buoyancy
can be visualized by reference to Figure 2.1-4. A rise in
the top appears to reduce seeding effectiveness (or E~nhancement
of precipitation efficiency), but since the overall condensation
(and hence PWC) increase s wi th depth, the net dynamic seeding
effect is posi t i ve. However, if there is a stable lE.yer alof t ,
as seems to be the more normal case in the Sierra, then there
results an enhancement of updraft leading to a wider dispersion
of the PWC in the outflow region, which effect is in keeping
with recent detailed 2-D numerical model results (Eirh-Yu
et a1, 1980).
In. t he case of bands the dynamic ef fec ts require more
complex mesoscale modeling procedures, and the working conceptual
model to be used herein is based upon them (Fritsh and Chappell,
1980; Cotton and Tripoli, 1980).
The entrainment factor used was roughly a dDubling of
the bubble area per two steps (1000 m) for a 1 kHl starting
radius. This is in line with general observations, but may
be low for Sierra storm convection. A 1 km starting radius
was used. The center of the outflow region was uS"lally 30-40
km downwind from the starting point, and 20 or so km downwind
of the lower level drift. Should there be a continuing updraft,
rather than a bUbble, then the precipitation particles falling
from the outflow region (which itself is centered 10 or more
km downwind of the higher steps) would fall wei] ahead of
that updraft. However, in low wind shear cases, g"raupel type
2-14
precipitation particles could fall into the updraft. If this
were the case, and a Hallett-Mossop type generation of 10
particles of 10-7g size for each graupel particle is permitted
in the -40 to -80 C zone, then the natural case would be seeded
by these additional particles. A calculation was made assuming
104 graupel particles per m3 were introduced, and all of the
previous runs recalculated. The seed-no seed differences
in PWC in the outflow zone were now very small, with some
negatives. The ice mUltiplication had the effect of overseeding
the long duration natural cases with little S-NS difference
resulting.
For intermediate durations the same was true, but for
short durations there was no overseeding effect; there was
substantial precipitation, and seed-no seed differences were
nil.
Overall, these calculations suggest that when this ice
multiplication system is fully effective, seeding will not
compete with it successfully. The key predictive factors
for this type of ice mUltiplication seems to be the wind shear,
the depth of convection and the air mass type. The latter
has not been precisely defined, however recent analyses of
the distribution of sounding-defined water saturated cloud
top suggests that it may occur most frequently when cloud
bases are low (high cloud base saturated mixing ratio). This
is in keeping with previous work relating cloud base mixing
ratio to the prevalence of large drops produced by the condens­ation-
coalescence process. Some preliminary analyses of water
saturated cloud top distributions indicate that there is a
potential for these ice multiplication effects when the cloud
base mixing ratio exceeded 6.5 g/kg, and according to the
sounding data this occurs nearly half of the time, although
2-15
,.,.
this may be climatologically slanted toward the tropical maritime
type air mass. However, unless fallback into the lpdraft is
assured, the result would be only to produce many small ice
cry s t a lsin the -40 to -8 o C range of any orogra:;>hic cloud
catching the fallout.
Tentatively, it appears that fallback into ~onvection
is assured for convection too shallow to extend m'~ch higher
than the _8°C level. This would be characteristic of embedded
band convection. However, for the deeper convection geometrical
factors preclude any fallback unless the vertical wind shear
from convection base to top is less than about 3 Ms- 1 • This
still leaves open the question as to the possibility of graupel
falling from the top or one cell reaching the lower levels
of another cell.
In bands there is a mesoscale circulation su.perimposed
upon the orographic flow, and containing individual eell circu­lations.
This would help to distribute fallout particles
to downwind cells. But these may be primarily d~'ing cells
lying in the mesoscale outflow region, and most likely are
already heavily iced.
In the C2 cellular case, there also appears to be a mesoscale
pattern, with a line of new starting cells lying in the foothills,
and older decaying cells drifting downwind up the slope. To
play an important role in the overall water balance, ice multipli­ca
tion would have to occur in the starting 'line.
Without fallback into convection, there would still be
the possibility of this type of ice multiplication within
any orographic cloud present. This would merely produce numerous
ice needles in the -40 to -80 C range, and could ~~~uce LWC
2-16
at lower levels. This development could account for the high
ice concentration wake that moves up streamlines as indicated
in Figure 2.1-1.
Considering all factors, it is presently estimated that
of the 50% or so cases where the air mass favors this type
of ice multiplication, 50 to 60% would result in graupel fallback
into active cells and resultant ineffectiveness of seeding.
Thus, cases favorable for seeding would be about one third
overall.
The delineation of areas of seeding response indicated
in Figure 2.1-4 and in the text (based upon cloud base mixing
ratio and wind shear values) are provided as elements of a
conceptual model. They can be used to form testable hypotheses.
It is conceivable that refined modifications of this model,
or use of different key inputs (e.g.; starting radius, entrainment
factor, cloud base, etc.), or entirely different models, can
supply somewhat different testable hypotheses within the same
key parameter framework. These should and can be developed
prior to commencement of an exploratory phase.
The model calculations provide a microphysical output
that can also be compared to observations obtained by aircraft.
Figure 2.1-5 is an example of the ice particle distribution
in the outflow zone for a single case. The solid line compares
well with the exponential type distributions observed. The
dashed curve is the ice water concentration per particle set,
i.e., the product of the paricle number concentration and
the mass. It is fairly level through a range, and this fits
observations in general. Curves such as these for various
parameter combinations, can serve as the basis for formulating
testable hypotheses. More importantly, the progressive change
2-17
-4 -3 -2 -I
LOG MASS (g )
-5
-
-
-
-
-
-
f---- ,
L. __,
L ___,
r----~ L.___,
~______...J
I
-I I I I I I
-6
4
2
LOG CONC
(NO M-3)
5
3
-3
-2 0
LOG CONC
X MASS
( gm-3 )
-I
Fig. 2.1-5 Distribution of particle mass.
2-18
in the distribution with time (or height of the bUbble) can
be predicted and tested. In the runs there was a bulge in
mass curve (dashed) that progressed to the right with time.
2.1.3 Combined Convection and Orographic Scale.
A further exposition of the conceptual basis for both band
and cell seeding is given by a series of seeding simulation
by the GUIDE model (see Section 4.2 for a description). Figures
2.1-6 to 2.1-13 show the results of seeding in a stable orographic
cloud with TB-1 flares in curtain mode from different heights
over the foothills, using different nuclei sources. The calcu­lations
are made only for curtain midpoint and to get a complete
effect per curtain several runs at different levels are needed.
Clearly for TB-1 the optimum fallout pattern over the barrier
occurs when the curtain is centered at 350 mb above the valley
floor. Figure 2.1-11 shows that moving the source back upwind
produces little change. Table 2.1-5 summarizes the figures.
Figures 2.1-12 and 2.1-13 pertain to different generator
types (which could be used to line seed, but not to produce
a curtain). Figures 2.1-14 through 2.1-16 show the production
curves (starred points) for TB-1, Aerosystems, and NAWC generators,
respectively. The TB-1 flares have, in general, the lowest
nuclei production per gram. However, the typical output in
grams per second is the greatest. The best comparison between
types is obtained by multiplying the production figures by
this output. The auxiliary curves on Figures 2.1-15 and 2.1-16
make this adjustment and show that the types are more nearly
equal in application than the vast differences that their
production curves would suggest (although unit costs to achieve
these production curves can vary substantially).
2-19
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Nucleating Coldest Precipitation
Figure Method X (km) Z (mb) Temperature °C (acre feet)
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2-28
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1_1 I I l I l l * * * * * * - ~I 1___ I I_I
-2.50£+01 -2.00£+01 -1.50£+01 -1.00£+01 -5.00£+00 0.00£+00
Temperature CQe)
Fig. 2.1-14 Nuclei production curve vs temperature, TB-1.
------------------------------------------- 1 1 1 1 1 1 1 1
J .bOE+OI - *
._----.1--1
_ -2.16
•
~ AEROSYSTEMS
*~ (.035 Gs-l) •
TB-l
(10 Gs-l)
• • * • * • • •
-----
ADJUSTED AEROSYSTEMS /
•
-- ..... ~---
* *
--- I. ",OE+OI •
I.IIOE+OI •
I.I0E+01 • \
\
I.OOE+OI -. \ -
1_1 __--_--__ 1_--------1_-----_--1_-------_1 ----1_--------1 -----1_--------1--- I I_I
-2.50E+01 -2.00E+OI -1.50E+01 -1.OOE+Ol -5.00f.+00 O.OOE+OO
1
1 *
1
1
1.50E+01 -
1.20E+01 -
@
I-<
bll
I-<
Q)
P<
If)
M
cd
~
If)
>.
t\j
I-<
I
U
W
Q)
0
U
H
Temperature (OC)
Fig. 2.1-15 Nuclei production curve vs temperature for TB-l and Aerosystems
silver iodide-acetone generator.
--J-------i-------..,..l--~----..-.-J----I------J---I---
l.bOE+OI - 2 • • • • • • • •
._----- 1 1
* •
-3.35
• • • *-
NAWC
-1
(.0045 GS )
•
•
./
• •
TB-l
(10 GS- 1 )
•
ADJUSTED ~
NAWC ~,
"\
\
-- ~ - --- .-. .----. ... ~
I.IOE+OI -
I.OOE+OI1-_*1 1 1 1 1 1 \ 1 -:...1 ., •1 • 1 I_I
-i!.SOE+UI -2.tJOE+01 -I,50E+UI -I,OOE+OI -5.UOE+00 .0.00£+00
1.20E+01 •
1.30E+UI • --. •
1.50E+OI •
1.40E+OI -
~
1;'0
I-<
(])
p..
1Il
...-i
cd
~
1Il >- I-<
U
(])
U
l\:)
H
1
W
......
Temperature COC)
Fig. 2.1-16 Nuclei production curve·vs temperature for TB-l and NAWC's
silver iodide-acetone generator.
Figures 2.1-17 through 2.1-20 show the results of GUIDE
runs for the convective case. Various convective illstability
tops are covered for the TB-1 curtain case, in which the mean
major band sounding was employed as an i npu t • F i gllre 2. 1-21
pertains to a C02 curtain, but allows it to be ef1ective for
a longer time than can be expected. In all cases the curtain
midpoint is allowed to remain within convection (often at
its top) for 2000 sec, and then allowed to pass into the environ­mental
cloud. A mean updraft LWC of 0.5 gm- 3 and updraft
speed of 0.1 mbs- 1 was employed.
Figures 2.1-22 through 2.1-24 pertain to grounc generator
seeding (NAWC type) of convective cells. The mean C-2 (2/3
cell coverage) sounding was used as an input. Table 2.1-6
summarizes the figures.
Figure 2.1-25 plots the fallout on horizontal (X-,Y) section.
It is clear that the model can be useful in posit'Loning the
release so as to target a specific area. This brtngs up the
very important matter that the upward-downward motion cycle
of nucleant and ice particles is helical in form.
The above Guide model runs are not intended to be detailed
guides for the seeding scenario. They only illustrated key
aspects of the conceptual model. The Guide model would be
used during the course of an operation to supply detailed
guidance as to seeding mode and procedures in a give!l case.
Guide does not give any but meager guidance on how to
exploit the dynamic effects potential of the unst~ble case.
The most likely place for seeding to generate an enhancement
of buoyancy, and therefore a dynamic invigoration and expansion
of the system, is in the high LWC area in the foothills (or
2-32
----------------------------------------------------------------- -----------------~- I I , I I I I I I I
~.OOE+02 - " "
._----- 1"
-
" 2- 7*3245. 5? 2 .-. 2
.:3. 2 *2** 24 *" " 0* r? ".2" *
*2" * *
* 3*
" " ~ *
-8.80
CIT 350 - * * * .. * 2 2 .. * .. 2 2 5***" 2?
" " * .. .. * * * 3 *2 3* 322 *2 **2 2
* * * * ..
15,300 ,,* * * *
* * * *
* " *
* .. " "
* " ..
"
" *
~ *"34 5 2*2
O.OOE+OO •1_.1_.-----I--. -_-_I * I - I * ~ -_I_~ 1 1 1 1 1_1
O.OOE+OO 2.QOE+Ol 4.80E+Ol 7.20E+Ol Q.ftOE+Ol 1.20E+02
1.00E+02·
5.00E+02 •
I
I1I
r-'"'\ 1+.00E+02 -
~
'--'
~ ...c:
b.O '3. OOE +02 • 'M
~
t'V
IWW
2.00E+02
distance (km)
Fig. 2.1-17 AgI curtain-convection, curtain 15 km downwind and 300 mb above
valley floor, top of convective instability at 350 mb above valley floor.
Sounding type - major band.
I I
-1--1-----------1----------1-----------1----------1-----------1----------1-----------1-----------------.~---------'
~.00£+02 • • •
'5.00£+02 •
-11.6
0
• •
165 AF
• •• 3 II
2 4.2 211 32 211 3. ~3
2 ••
l\J
IW
~
r-..
~
'-'
~
oM
~
~.00E+02 •
3.00£+02 •
2.01)£+02 •
\ .00£+02 •
CIT 400 • •• • 2 2 :5 3 :5
• • • 2 • •
• ••
• • • •
• • • •
15,300 •• •
•
•
• • •••
•• • •• •
• • •
•
•
4 •
·22·'5.3 ••
• 2.23.2 2 ••
2·22 ••••••
··2.22 ••
3
1
23 22 2 • 2 1
2·2 • 1
2 •••• 2 • 1
O.OOE+OO •~. *
1_1 --__ 1_--------1_-----_--1_--__--__ 1 --__--1 --__ 1 - 1 1___ 1 ' __ 1_1
0.00£+00 2./lOE+01 4.80E+01 7.20£+01 Q.bOE+01 1.20£+02
distance (kIn)
Fig. 2.1-18 AgI curtain-convection, curtain 15 kIn downwind and 300 mb above
valley floor and top of convective instability at 400 mb above valley floor.
Sounding type - major band.
* •
i-I---------I---------I---------I---------.--------~---------.----------1-------------
~.OOE+OZ • * *
distance (km) •
3
2. '5 **2 2 **2 q
2 2 *3 3 *36 •
*
-16.70
* * * 2 2 2* 2* *2* *] * 4 *.
* * 2 * 2 * 2 * 2 * 2*2 **'5 **
* * * ***2
* * *
2 * * * * *
2 * * *
2 * *
* **
* * *
** * *
2 *
*
*
*
* *
*
* **
*
* *
* *
* *
CIT 450 *
* *
•* *
*
~.00E+02 • 15,300 u
* O.OOE+OO ~1_1~--_1 __-- 1 --*-_--1 1 --1 ----1 1 * 1___ 1 1_1
O.OOE+OO 2.QOE+01 4.80E+01 7.20E+01 Q.60E+01 1.20E+02
1.00E+02 •
2.00E+02 •
5.00E+02 •
4.00[+02 •
,.--..
~
\.-J
~
.,-i
]
I:\:)
I
CJJ
C.1l
Fig. 2.1-19 AgI curtain-convection, curtain 15 km downwind and 300 mb above
valley floor, top of convective instability 450 mb above valley floor.
Sounding type - major band.
1-.---------.---------.---------.---------.---------.---------.---------.----------.---------
6.00E+02 - • * ..
».00£+02 - CIT 500
r-..
~. ~.00E+02 -
\.....J I " I • ~ ..t::. I •
bL)
.r-4 j.OOE+02 - 15,300 •• ~
l'V
I .2.00£+02 - VJ
Q)
1.00E+02 -
O.OOE+OO -l_l l l I I ~ I I ~I---__----I---------I---------I-I
O.OOE+OO 2.40E+01 4.80E+01 1.20E+01 q.~OE+01 1.20E+02
distance (km)
Fig. 2.1-20 AgI curtain-convection, curtain 15 km downwind and 300 mb above
valley floor, top of convective instability 500 mb above valley floor.
Sounding type - major band.
* •
I-I---------I---------I---------I--------- --------~---------I--------~-------__.___ ------------
6.00[+02 • * *
5.00E+02 -
4 *
*22**5*3 ***
* 2**3 *2 2 * *
2**22* 2 ** 23 22 2 * *
**2*22 2***2 * * 2
***** 2 * **
-l1.ao
* * * 3 II
CIT 400 * ** * 2 2 3 3 3 2 4 *2 24 32 24 3* B
* * * 2 * * 2 * *
* **
* * * **
* *
15,300 ** * * * *
* * * *
O.OOE+OO •I_I I_~ I I - I I I 1 1___ 1 1_1
O.OOE+OO 2.40[+01 11.60[+01 7.20E+Ol Q.bOE+Ol 1.20[+02
1.00E+02 •
r--- .\.00[+02 - ~
'-'
~
~
.~ 3.00E+02 •
Q) ..c::
l:V
Iw
-..J 2.00E+02 •
distance (kIn)
Fig. 2.1-21
valley floor,
SOWlding type
C02 curtain-convection, curtain 15 kIn downwind and 300 mb
top of convective instability 400 mb above valley floor.
- major band.
above
-------------------------------------------------------------------------------------- I 1 I 1 1 1
6,OOE+02 •• • • •
• • •
0.1 AF*
NAWC
* (SOME TRAJECTORIES f\OT RUN
LONG Ef\OUGH TO REACH GROUND)
• • o ,OOE+OO - ~ •
1_1 --_1 -- 1 1 1-- --1 --- I ~I I I_ I_I
O,OOE+OO 2.QOE+01 a,AOE+OI 7,20E+0\ q.bOE+O\ 1.20E+02
1,OOE+02 -
5,OOE+02 ·
r-..
'@ '--' ",OOE+02 · CIT 350 -8.9°
I • * * *;0 .. •
~ I ~. 2 ••
'"Sn I • • 2 2 2 2 • ? • •
oM I • • • 2 • CJ) .r:: 3.00E+02 · • • • • •• 2. 2 2.
I • • • • •
t-:l I • • • • • I 1 • VJ
00 I • •
2,00[+02 · ••
distance (kIn)
Fig. 2.1-22 AgI point source-convection, ground source 20 kIn downwind and
40 rob above valley floor, top of convective instability 350 rob above valley
floor. Sounding type - Cl.
,-a---------,---------.---------,---------i---------,---------.---------a---------.---------r----------I--I
6.00E+02 .. • " ...
3.00E+02 ..
-13.9°
" "*
•
" "
"
I11
2 3 • I
* 2. 2 l' ..
2 " 2 • •• 3
" 2
•
" "
"
* 2 *2
"
"
"
""
"
* "
12 AF*
NAWC
*(SOME TRAJECTORIES NOT RUN
ENOUGH TIME TO REACH GROUND)
•
• " .
*•
2
•
*
•
• •
2 2 *3
, 2 2 2 A AA •
2.00E+02 ..
1.00E+02 ..
O. OOE +110 ..
5.00E+02 ..
II
r--'\ . I
~ I
~ 4.00E+02 ..
~
'r-!
]
tv
I
CJJ
(,0
1_1 1 1 1 1 1 1 1 1___ I I_I
0.00£+00 2.QOE+Ol 4.60E+Ol 1.20E+Ol q.&OE+Ol 1.20E+02
distance (km)
Fig. 2.1-23 AgI point source-convection, ground source 20 km downwind and
40 mb above valley floor, top of convective instability 400 mb above valley
floor. Sounding type Cl.
1-1---------.---------1---------1---------1---------.---------I---------.---------.---------~--------s_I
6.I)OE+02 - • • .-
5.0 OE +02 - -17.5°
~ I CIT 450 ~. 2· ••
~ 1 • • • • :3 • • 2
'--' 1 • • • • • 2. • :3 :3 :3 q
~ 4.00E+02 - • • • • • -
-Sn 1 • •
1 • • • 'M
(1) 1 • • • • .J:: 1 • • • • • •
3.00E+02 - • • •
I • • • • •
1 • •
I:\:l
I~
2.00E+02 - 0
1.00E+02 -
O.OOE+OO -1_1 1 1 1 1 1 I ~I I___ 1 1_1
O.OOE+UO 2.QOE+O\ Q.80E+O\ 7.20E+01 q.&OE+O\ \.20E+02
distance (km)
Fig. 2.1-24 AgI point source-convection, ground source 20 km downwind and
40 mb above valley floor, top of convective instability at 450 mb. Sounding
type Cl.
Table 2.1-6. Summary of unstable Guide runs.
Coldest Precip-
Nucleating CrT Temperature itation
Figure PET Method X (km) Z (rnb) (rnb) -(DC) (acre feet)
Curtain:
2.1-17 Band TB-1 15 300 350 - 8.8
2.1-18 Band TB-1 15 300 400 -11.6 165
2.1-19 Band TB-1 15 300 450 -16.7
2.1-20 Band TB-1 15 300 500 -21. 6
Ground:
2.1-22
2.1-23
2.1-24
Cell
Cell
Cell
NAWC
NAWC
NAWC
20
20
20
40
40
40
2-41
350
400
450
3*
* SIGNATURE
* *
*
*
~l---------I---------I---------I---------I---------I---------~-------~--------~---------~--------~
5.115E-01 - J FALLOUT TRAJECTORIES
I * *.
1
1
I .
3.1IE-Ol •
,-...
! I.Cl1E.OI·
~O
'M
(1) ...c: l.l5E-00·
'\. FOLSOM D LAKE TAHJE
I.\:l
I
I*.\":l
-1.5lE-0 1 -
-S.lbE-Ol -
-5.00E-01 -l_l* I * * * * q • I_~ I I l I 1 1 1 1_1
o.OOt_oo 2.QOF+01 Q.AOE_OI 7.20E_01 q.~Ol.OI 1.20E.02
distance (kIn)
Fig. 2.1-25 AgI point source-convection, xy plot. Sounding type C1.
in the most active portion of bands). There is a suggestion
that C02 seeding might be more effective than AgI in developing
a dynamic response.
2.2 Hypotheses and Response Variates
In consideration of the existing network, and of past
successes and failures in observing individual bands and cells,
it would appear that the use of a band as a unit of observation
is feasible, while that of a cell is problematical, and possible
only if very precise aerial seeding is practiced. In the ground
seeding scenario, the lack of control precludes the individual
cell approach and a three-hour (or longer) period of area-wide
cell seeding by a ground generator network is recommended.
Table 2.2-1 summarizes briefly the status of SCPP with
respect to observations along the links in the chain from
curtain to ground. In the case of ground generators the sampling
of the "curtain", in this case a psuedo-curtain generated
from a line of generators and being entrained into the inflow
zone, the measurements are far from precise. There are also
great differences between convection types. Specifically,
the observation of interactions between the curtain's seeding
material and the cloud is much more easily observed in the
case of cells than in the case of bands. Individual cells
can be clearly observed and penetrated with precision, whereas
cells in bands are obscured by cloud and difficult to find.
It is no wonder that situations of high water content have
been more frequently observed in cells than in bands; this
condition reflects ease of observation.
Aerial observations in the past have tended to concentrate
in the -50 to -100 C zone, where most of the calibration seeding
2-43
Table 2.2-1. Links in seedin~ chain.
Link Name
1. Curtain
2. Fallout
(aloft)
3. Precipitation
on the ground
4. DYnamic effects
5. DYnamic effects
(ground)
How Measured (both directly
and indirectly)
• Wyoming AC
• Radar
• Sounding data and Guide
Model
• Sounding data and Guide
• Unexplored radar
possibilities
• Precip. gauge network
• Ground level microphysics
• Radar for horizontal spread
and/or top rise
• 15 min. precipe gauges
• 15 min. meso-net
• Ground microphysics
How Precis~?
Moderate tJ good
Dispersion aspects fair
to poor
Fair to moderate
MOderate to good
Fair
Good for bands
Fair to poor for other
convection
has been done. The HUBBLE runs demonstrate the importance
of making observations through an extended vertical profile.
The profile should penetrate individual cells so that microphysical
properties at various levels in a given cell can be compared.
This requires flight procedures different from the one level
tracking by means of a "homing pigeon" device which has been
used appropriately for the seeding calibration I'uns. When
this approach is used in a convective draft the seeding material
and associated microphysical particles moving Lpward past
the sampling level is being observed in seeded eells, and
its interpretation is thus complicated.
2-44
At this point it should be reiterated that a shallower
curtain is recommended. In fact, the "curtain" generated
after a period of an hour by a moving point source would be
sufficiently deep for proper entrainment into the inflow zone.
This procedure would reduce the amount of redundant seeding
material and sharpen the analysis. Under this plan the seeding
would normally be conducted at temperature levels warmer than
the -50 to -lOoC zone, so that seeding would only occur where
the nucleant is entrained. It would be virtually impossible
to conduct this type of seeding by injecting the material
directly into selected cells within a band (although it could
conceivably be done in widely spaced individual cells). The
procedure for bands would be directed toward treating the
entire width of the band along a sector that is tens of kilometers
long. In the case of ground seeding the line of continuous
generators would seed numerous cells over an extended area.
In either case, aerial sampling (which could not possibly
cover the entire treated area) would concentrate on convection
that is predicted to be well targetted.
Table 2.2-2 lists in summary form the various response
variates, comparison analyses, and hypothesized effects of
seeding for major bands. It is assumed that the seeding would
be carried out as discussed above, with three-way randomization:
AgI, low C02, and placebo.
Table 2.2-3 summarizes similar information for the scenario:
cells, ground generator seeding. Here it will be noted, the
aerial observations and radar can define individual cell behavior,
however, the ground level observations (with the probable
exception of microphysical) define only observations in a
treated and a comparison area.
2-45
Table 2.2-2. Major bands, modified curtain seedir.g.
Response Variates
Vertical profiles of
LWC, IWC, ~VC, vertical
velocity across seeded
segment. SlITIilar
observations outside
of treated segment.
Radar (mostly for
fallout echoes)
Echo dBz, top, width
Ground level precip,
pressure, wind per­turbation
(15 min.
resolution) and
microphysical
perturbation
Comparison Analyses
• Inside/outside of
treated band segment
• Seed/placebo in
treated segment
• Inside/outside of
treated band segment
o Seed/placebo
• Inside/outside of
treated band segment
• Seed/placebo
2-46
Hypothesized Effects of
Seeding
• Model predictions using
depth, ur:draft, cloud
base, mixing ratio, wind
shear, ard other
parameters.
• Top rise
• Band broadening
• Hore inte:nse perturbation
• Broader perturbation
• Crystal r~bit change
• Increase in particle
concentration
• Decrease in particle
size
Table 2.2-3. Cells, ground generator seeding.
Response Variates
Vertical profiles of
LWC, IWC, PWC, vertical
velocity in cells.
Periods covered
2-3600 sec.
Radar (mostly tracking
treated cells)
Echo dBz, top, width
Ground level precip.
and microphysical
perturbations
COmparison Analyses
• Inside/outside of
selected (obs.)
treated cells
• Seed/placebo
• Inside/outside of
cells
• Seed/placebo
• Inside/outside of
treated cells area
• Seed/placebo
2-47
Hypothesized Effects of
Seeding
• Model predictions using
depth,-updraft, cloud
base mixing ratio, wind
shear, and other
parameters
• Top rise*
• Cell broadening*
* collectively and
individually
• Hore intense area
precip
• LonRer duration area .:> precip
• Change in crystal habit
• Increase in particle
concentration
• Decrease in particle
size
2.3 Seeding Modes and Procedures
2.3.1 Major Bands. It is considered feasible to
seed major bands with aircraft using at least three seeding
systems: dry ice curtains, silver iodide curtains produced
by dropping flares, and silver iodide line seedin~ produced
by using acetone generators similar to that used in Santa
Barbara II, Phase II. In addition, major bands with roots
of convection reaching to foothill elevations are deen~d seedable
with ground generation techniques (pyrotechnics or acetone
generators as was performed in the CENSARE project in the
Sierra Nevada). As will be documented later, it is fel; desirable
to maintain a three-way randomization scheme testing C02'
silver iodide, and placebo seeding of major bands.
The primary consideration in terms of where to seed in
major bands becomes a question of where liquid water il; routinely
produced. Analyses of SCPP data and the earlier DRI flights
in the SCPP area, and a comparison of Sheridan versus Freshpond
rawinsonde data with respect to water saturation all suggests
that liquid water is generated in the foothill region of the
Sierra Nevada during winter storm periods. Additionally,
regions of liquid water with low ice crystal concentrations
appear to be frequently related to developing or new growth
convection towers. Limited analysis of SCPP data f)r a major
band (Moore, et al., 1980) indicate in one case tha: the major
updraft region in higher liquid water contents were to be
found a t the rear of the major band (see Figure 2.3··1). There
is a suggestion that other major bands may be froat feeders
instead (SCPP analysis conference, Oct. 21-23, 1980), although
there has been the suggestion of conversion of froat feeding
bands to back feeding bands as the band encounters the foothill
region of the Sierra Nevada. Additional studies are warranted
2-48
6e+4
6e+2
6e
~
=> BAND
MOTION
-5°e -- J
ooe
(&)
7 7 71/77777777717777
~---30 -50 km -----~
Fig. 2.3-1 Convective band schematic. Wind barbs shown at 2, 4,
and 6 km, respectively, are band relative. Speed is in knots.
Schematic is representative of a band in the valley.
2-49
to develop the capability of determining the location of this
updraft region within major bands hopefully through €'xamination
of radar returns (conventional and/or doppler), although direct
measurement of this region may be necessary either by the
seeding aircraft equipped with a JW instrument or a supplemental
cloud physics aircraft.
In the case of dry ice or silver iodide curtaj.n releases
in major bands, it is proposed that these curtains bE! ini tia ted
at the -10o C level. Figure 2.3-2 provides a mean sounding
for major band occurrences as documented by Electronic Techniques
Inc. (ETI). On this mean sounding the -10o C levl~l is near
12,SOO feet MSL. If the dry ice and flares drop approximately
4000 feet, then a curtain should be produced from ~bout -10oC
to -3 0 C level (approximately 8S00 feet MSL). 11 the case
of an acetone generated line source it is proposed that such
a source be genera ted at ei ther the OoC or -SoC leve:. depending
upon terrain clearance considerations and possibly icing consider­ations
in flying for long durations within convection bands
of this type. The advantage, or course, of flying at the
OoC level is that ice should not accumulate on the aircraft.
For the mean sounding, this would either be 7S00 feet MSL
or 10,000 feet MSL. Assuming an average updraft strength
of 1 m/ sec, the m'a terial would rise from the OoC to the -10oC
level in about 13 minutes and from the -SoC to tile -10oC in
about 8 minutes.
Seeding of major bands would commence in secto:s of baads
as they move eastward from the Sacrmento Valley area into
the SCPP experimental area. It is proposed that seed:.ng commence
over approximately the 1000 foot contour in the footllill region
and terminate at the 4000 foot contour which rep:resents the
western boundary of the SCPP experimental area. The lower
2-S0
l:\:)
I
U1
f--l
f:>0
WINO SCALE
400
700' 7" / 7" / / / / / ~ / / / / / /. / 4 J 1700
800/ / / / / / / / / 7{\ / / / / / / J /+ V leoo
900 / / 7' 7' / / / / / /) \: / / / / / / t % • / I
I000 ,./ ,./ 7 / 7 / 7 / 7 " 7 " 7 " 7 " 7A' 7 / 7 / 7 / 7 / 7 / >Vr f 7 / ~ 711000
Fig. 2.3-2 Mean sounding for the 1980 forecaster designated major band echo type (PET).
region that this seeding would embrace is favored because
it is likely that the additional orographic indueed uplift
will enhance the production of supercooled water and thus
the seedability within this zone. Seeding further away into
the valley itself would compound the targeting of mi~rophysical
seeding effects in the SCPP experimental area. Seeding past
the 4000 foot contour is not deemed as warranted since this
region appears to be one that is experiencing increased production
of ice and reduced liquid water contents (based upon SCPP
and earlier DRI flights). This procedure will 8.1so allow
pre-seeding alert of the seeding aircraft crew since major
bands normally travel across the valley and some pre-seeding
sampling of the band over the Sacramento Valley.
It is assumed that the GUIDE model would be available
for real time decision making and that it will l>e used to
assist in targeting of the microphysically induced effects
of seeding either with dry ice or silver iodide curtains such
that the mid-point of these curtains would be t.argeted to
pass over the Central Sierra Snow Lab. This apprc,ach should
also produce microphysical effects in the data dense instru­mentation
network along 1-80. As a part of the C:rUIDE model
calculations, consideration is given to the backing and then
veering of the steering level winds as the air mass approaches
and then goes over the Sierra Nevada barrier. In this regard,
it is conceivable that the GUIDE model might suggest seeding
some distance further south than might initially be expected
by examining say the 700 mb wind flow at the Sheridan rawinsonde
observation site. It is proposed that seeding would be conducted
along a 30 km sector of the band and that this sector would
be, as mentioned before, centered to affect the Central Sierra
Snow Lab. Continuous seeding in the updraft region of the
major bands is proposed from the initiation point of seeding;
2-52
i.e., the 1000 foot to the 4000 foot contour level or a distance
of about 45 km. It may be advantageous to develop some termination
criteria such that a major band that is initially seeded is
discontinued whenever the maximum dBz level along the band
in the SCPP vicinity drops below some minimum dBz value such
as 20 dBz.
The horizontal pattern produced by aerial seeding of
major bands would appear to be a zig zag pattern if it were
plotted out schematically. The expansion in the curtain or
line generated curtain would of course expand with time.
As Vardiman has pointed out, the overlapping of curtains is
assisted by speed shear between the top and base of the curtain.
A seeding aircraft flying at 150 knots would produce curtains
within bands along a 30 km seeded sector such that without
any speed shear the plumes would probably not merge over the
SCPP experimental area if the horizontal dispersion in bands
was approximately 1 mjsec or less. With a 1 km curtain and
seeding over Folsom with a 5 kt speed shear, the plumes would
overlap in the 4000 foot contour region only if a new seeding
curtain was generated approximately every five minutes - not
practical at 150 kts over a 30 km sector which occupies approx­imately
10 minutes. If the shear were ten knots, then plumes
would overlap with curtains generated every 16.7 minutes ­within
the time available. As seeding progresses up the barrier
as a band moves eastward, the plumes would become less prone
to overlap due to the shorter distances between release and
the 4000 foot contour level. They would tend, however, to
overlap further downwind over the upper part of the barrier.
Figure 2.3-3 provides a schematic of what the flight tracks
might resemble for a typical operation.
2-53
tv
I
CJ1
w::.
YUBA CITY
N+
~
o JACkSON
o 5 10 2,0 '3,0 4:' kiLOMETERS
I , , 20
? 5I 1I0 I NAUTiCAL MILES
Fig. 2.3-3 Possible seeding pattern in a major band.
The GUIDE model has been utilized to simuate the micro­physical
seeding effects of a major band with varying seeding
modes and input sounding data. Dry ice, silver iodide curtains,
and silver iodide line source releases were all simulated
for varying depths of convection and h€ights of seeding.
Figures 2.1-6 through 2.1-25 contain these plots. It appears
seeding with all three generating systems would pr~uce seeding
effects on the upwind side of the barrier. By using the GUIDE
model to target the microphysical effects centered on Central
Sierra Snow Lab, taking into consideration the backing of
winds with progression up the barrier, the 30 km long sector
should allow seeding effects to be detected along the 180
network as well. Any additional dynamically produced seeding
effects are anticipated to occur to the right of the steering
level flow (as in NAWC's Interim Report No.2) such that these
effects should be detectable in the central and southern portions
of the SCPP experimental area.
A cost comparison of dry ice and the two methods of silver
iodide seeding is as follows: Assuming an average band movement
equivalent to approximately one half of the 700 mb velocity
noted by ETI in bands would be 10 m/sec. The total time of
seeding between initiation of seeding in the 1000 foot contour
region to termination at the 4000 foot contour should occupy
(about 45 km east of initiation) 75 minutes. Allowing 7 minutes
seeding time and 3 minute turns, this translates into approximately
seven seeding curtains when flying at 150 knots. During this
period, 21 kg of dry ice would be used (at the seeding rate
of 100 g/km), 420 flares (spaced every 500 m), or 2-3/4 gallons
of 2% silver iodide solution (or about 150 g of AgI) would
also be used. The approximate cost of seeding one major band
for these three modes would be $30, $8400 ($20/each), and
$100. The output of the acetone generation system could easily
2-55
be doubled if desired in terms of seeding rate considerations
by burning two generators simultaneously.
2.3.2 Ground Seeding of Convective Cells. The utility
of ground generators to seed convective cells (either Cl or
C2) depends upon a number of factors. Primary considerations
include the relative instability of the atmosphere, the temperature
of the air mass, and prevailing wind directions and speeds.
Of perhaps primary importance is the stability of the atmosphere.
Under stable conditions the entrainment of effluent from ground
generators into active growth regions may be restricted.
In the case of PETS Cl and C2 , we know from ETI'~ work that
these PETS typically occur post-frontally and are corrEspondingly
normally unstable in character. Figure 2.3-4 (j:rom ETI I S
Interim Progress Report dated July 1980) illustrates the average
conditions. This figure indicates a region of instability
beginning near 1000 m for C2 and near 500 m for C1 ca.ses.
It is assumed that any seeding from the ground would
be performed with silver iodide dispensers. Dry ice does
not ofer a tractable capability from the ground and organic
generating systems are still in the prototype stage (If develop­me
n t. Due to the we 11 known temperature dependency of silver
iodide in"terms of the production of active ice nuclE~i, seeding
material must be transported aloft from the surface to reach
effective levels starting near the -5 to -6o C level. From
Figures 2.3-5 and 2.3-6 (also taken from ETI's Inter:Lm Report)
i tis seentha t the - 50C 1eve 1 for C1 's is near :~ 5 0 0 mand
2800 m for C2 's. Radar climatologies of the SCPP have cemonstrated
a tendency for cells to form over the foothill location (approxi­rna
tely 300-1000 m) and to move up the barrier and weaken (Suther­land
et al., 1978).
2-56
9
CI C2 CS MB EB AW/OR
E--IOOK~
7
2
8
3
6
.­I
(!)
w 4
I
O....J...__......1- ..I.-__--L__..I.-_-L__..I.- _
Fig. 2.3-4 Mean vertical profiles of equivalent potential temperature
(solid lines) displayed with the low level maximum 8e isotherms (dashed
line) by forecaster designated PET in 1980.
2-57
/Of) f)0 ~ ,," ,,0
l.'V
I
01
OJ
700
800/ , , , , / / ,..... / \ / / / / / / / t /1 j/ 1800 > > > > • • • c: • • • • • • > >
900
/ / / / / / / / / .X\/ / / / / / ~'OOO 1000 • • • • • • • • • • • > •
Fig. 2.3-5 Mean sounding for the 1980 forecaster designated cellular echo type (el).
400
WIND SCALE
ri> ",f) ",0 'Of) ~ .,f) f)0 ",f) ",0
~
I
CJ1
(!) 700///,/././.AL-A/././././,/,/i.XKl700
800/ £ £ £ £ £ , ,
7' \: 7"" 7
,
7
,
7
, , , , , , , , , / t /+ j/ 1800 7 7 7
900
1000/ ,L ,L ,L ,L >L ,L >L >L >L l;r >L >L >L >L >L l4ilJ1000
Fig. 2.3-6 ~1ean sounding for the 1980 forecaster designated cellular echo type (C2).
Based upon the above characteristics the follovling ground
seeding mode for convective cells is recommended. Multiple
lines of remotely controlled ground generators should be estab­lished
generally in different elevation zones. Fo.r example,
three lines of generators could be installed along the 300,
900, and 1500 m contours. Such an array is depicted in Figure
2.3-7. Spacing between generators would be on the order of
5 km. The lines would be situated such that when prevailing
wind flows are considered with C1 and C2 types, the expected
fallout of augmented precipitation would occur over the northern
portions of the SCPP experimental area. The Guide Hlodel would
be utilized to develop a climatology of this area for siting
guidance. The rationale for multiple lines arises f)'om expected
deviations on individual cases from the mean soundings ]~epresented
in Figures 2.3-5 and 2.3-6. If the level of convective instability
is higher than the norm, higher elevation genera":ors could
be utilized if they are indicated to be above an~T low level
inversions or isothermal regions provided wind speeds are
not excessive. The Guide model should be made available for
real-time seeding guidance to assist in targetting the effects
of seeding.
2.4 Estimate of Natural and Augmented Precipitation and Frequency
of Occurence
2.4.1 Major Bands. Data furnished by Atmospherics
Inc. (AI) provides a mean hourly precipitation rate oj' 2.2 mm h- 1
for the periods when major bands were the dominate echo in
the American River Basin during the 1979 and 1980 ob:;erva tional
seasons. These data are for non-zero precipitat:Lon cases.
Information from ETI indicate a mean duration of 3.43 hours
for major bands in the American Rivee Basin from radar data
(one-third of the 1977-78 season and all of the 1978-79 and
2-60
t\J
I
O'l
......
YUBA CITY
N+
o JACKSON
o 15 10 2,0 3,0 "p KILOMETERS
I , I 20
o, 1,5 1I0 I NAUTICAL MIL!S
Fig. 2.3-7 Possible remote controlled ground generator network.
....~ .
1979-80 data). Consequently, a mean natural prl~cipitation
amount for a major band occurrence is calculated to be 7.5 mm.
The seeding potential according to Sections 2.1 and 2.2 is
100% or more increase in about one third of the ~ases. The
band precipitation to be measured as a response variate is
probably about 7.5 mm per band, lasting about one hour, with
a coefficient of variation of about 1.
An examination of PET occurences during tne 1979 and
1980 opertional periods indicates 10 major bands Ln 1979 and
21 in 1980. If an average of 15 bands for the ~~-1/2 months
sampled is considered, then a total estimate of the number
of major bands per operational season would be 50 per year
(twice the number for day/night operations and 1.67 times
to account for a full season of operations).
2.4.2 Convective Cells. Data furnishei by AI for
C1 and C2 PETS combined indica te a mean hourly p::-ecipi ta tion
rate of 1.64 mm hr- 1 for the periods when C1 and C2 were the
dominant echo during the 1979 and 1980 seasons (these data
exclude zero precipitation events). Informati)n supplied
by ETI indicates a mean duration for C1 and C2 P.~TS combined
of 6.04 hours. Consequently, a mean natural pr('~cipitation
amount for C1 and C2 occurrence is calculated to be 9.91 mm.
The seeding potential according to sections 2.1 and 2.2 is
100% or more increase in about one third of the cases.
I t is estimated that 29 separate cases of C1 and C2 occurred
during the 1979 and 1980 observational seasons. If we use
an average of 15 cases per year, double that number :Eor day/night
operations, and mUltiply by 1.67, we reach an estimate of
50 cases per operational season.
2-62
2.5 Gaps in Knowledge
We need a better understanding of where the updraft regions
are located within major bands. It is hypothesized that these
regions will contain the highest liquid water contents and
will therefore represent the desired seeding regions. Techniques
of identifying these regions in a real-time mode are needed
(for both day and night operations). Remote sensing techniques
applicable to this problem should be considered if at all
feasible.
If ground generators are to be considered in an exploratory
phase of the experiment, then additional information on required
purge times is needed. This information will be required
to separate experimental units such that not-seed events remain
uncontaminated.
2-63
3. TASK FORCES 6 AND 9
Once the decision had been reached to suspend most field
activities on the SCPP for the 1980-81 season in order to
concentrate on detailed data analysis, nine different task
forces were organized to perform this analysis work. A lead
scientist was designated within each task force to coordinate
the analysis and reporting of results. NAWC was assigned
two task forces in which lead scientists were provided - Task
Force 6 - Robert D. Elliott "What is the opportuni ty for dispersing
seeding material by PETS and what changes in amount, duration,
intensity, and distribution of precipitation would be produced
by seeding as a function of PETS?" and Task Force 9 - John
A. Flueck "What are '~he major statistical components of an
exploratory experiment as a function of PETS?" Team members
on these two task forces are provided below.
Task Force 6
Brooks Martner (UW)
John Marwitz (UW)
Bill Moninger (NOAA)
John Flueck (FA)
Jim Humphries (Bureau)
Mark Solak (Bureau)
Rick Stone (DRI)
Rand Allan (AI)
Ron Stewart (UW)
Task Force 9
Robert Elliott (NAWC)
Don Griffith (NAWC)
Larry Vardiman (Bureau)
John Marwitz (UW)
Owen Rhea (ETI)
Key: AI
Bureau ­DRI
ETI
FA
NAWC ­NOAA
­UW
Atmospheric Inc.
Bureau of Reclamation
Desert Research Institute
Electronic Techniques Inc.
Flueck Associates
North American Weather Consultants
National Oceanic and Atmospheric Administration
University of Wyoming
3-1
Final reports were generated by each of the task forces
for presentation at a SCPP analysis conference held in Denver
on May 19-20, 1981. Section 3.1 and 3.2 provide these final
reports for Task Forces 6 and 9, respectively.
3.1 Task Force 6 - Transport and Diffusion
Cloud seeding technology attempts to change the water
balance of a natural precipitation system in sueh a way as
to alter or enhance the precipitation therefrom in a desirable
manner. In order to properly accomplish this, it is necessary
to 1) understand in some physical detail the natural process,
and 2) understand the modification of it. The scope of this
understanding must extend well beyond the mere detection of
a potential for modification. It must include the physical
details of the transport and diffusion of the seeding agent,
the nucleation process, the growth of ice crystals within
a seeding II signature", and· the subsequent fallout of precipi tation
size ice crystals to the ground. In this section a numerical
"Guide" model is discussed that embodies within itself these
details, organized so as to provide a practical aid i~ operations
and evaluation.
The basic concept of water balance in an orographic cloud
in which there is no convection is presented schematically
in Figure 3.1-1. The cloud is shown extending upwind in an
"area-wide" mode; however, the cloud on many occasions appears
only over the mountain. A set of streamflow channels indicates
the speed-up of air flow normal to the barrier, while the
sloping channels depict the fallout of an average particle
on its growth and descent from initial nucleation at high
levels in the cloud. Some significant variations from these
idealized patterns will be discussed later. A1BO shown by
3-2
FALLOUT CHANNELS
I
120
I
90
I
60
I
-30 o 30
en
"-J
LIJ
ZZ~
J:
() --.
3t
0 --. -J u..
DISTANCE (km) ~
Fig. 3.1-1 Orographic cloud concepts.
C
D
LW ­SD
condensation zone
depletion zone
liquid water accumulation zone
saturation deficit zone
dashed lines is the pattern of concentration of supercooled
liquid water, with a peak near the ground at the crest. Several
numerical models support this type of pattern which results
as the upslope advection of low level condensate and its production
by lifting exceeds its depletion by growing precipitation
particles in this region. Limited observational evidence
supports this for SCPP at present.
A numerical version of this conceptual model was constructed,
using the same cloud physics and treatment of fallout as was
used in Guide. A run was first made for a typical orographic
cloud, then the nuclei content at cloud top level was enhanced
3-3
five-fold. to simulate a broadscale seeding effect. The results
are summarized schematically in Figure 3.1-2. ~he figures
at the top of the fallout channels are the nuclei concentration
enhancement factor for that channel. The ones at the bottom
are the precipi ta tin enhancement in rnrn hr- 1 for tlla t channel.
In addition, seeding reduced particle size so that the channels
were shifted downwind at low levels as indicated by the arrow.
This is the "redistribution" effect of the seeding which under
extreme conditions leads to overseeding (i.e., that is to
the downwind transport of ice particles to the lee side evaporation
zone). This conceptual model provides a basis for a seeding
enhancement potential. However, there is a long stip between
establishing such a potential on the basis of orog::-aphic scale
water balance considerations and the implementa-~ion of the
required enhancement of the nuclei concentration. This latter
involves both the transport and diffusion of finite sized
emissions of nucleant from a fast-moving aerial sou:rce.
Fig. 3.1-2 Precipitation change in fallout columns with
seeding (rom hr-1).
3-4
Starting with the problem of transport, it is necessary
to know the air flow at any level over the barrier. In practice,
the main source of information concerning the airflow in SCPP
is the upwind sounding taken every three hours at Sheridan.
The air flow undergoes significant modification on passage
over the barrier. First, there is the acceleration of the
normal flow over the barrier as depicted in Figure 3.1-1.
In a neutral atmosphere potential flow theory calls for an
exponentially decaying perturbation to extend upward to infinity.
In the real atmosphere neutral conditions disappear at the
tropopause with a stable "lid" above. In winter storms the
lid where the flow becomes flat appears at about the 400 mb
level. But if the atmosphere beneath is stable, as in the
"stable orographic" cloud case, the flow crest tilts upwind
aloft from the terrain crest in the manner shown in Figure 3.1-3
This is called for in theory and is supported by analyses
of aerial observations of the King Air and the sounding cross
o
I I
30 60
DISTANCE (km)
I
90
I
120
Fig. 3.1-3 Typical stable flow pattern.
3-5
section analyses (Sheridan, Freshpond, Reno) by ETI. More
analyses may reveal variations with the degree of stability
and the strength of the basic normal flow, as is expected
also from theory.
In addition to the variations in the normal flow over
the barrier, there are quite severe variations i1 the flow
parallel to the barrier. The existence of a foothill "blocking
flow" directed north-northwestward parallel to the Sierra
Nevada range has been known for many years. CENSARE Jbservations
showed the development of a high surface pressure "dam" along
the foothills that deflects approaching air northwarj. Analyses
·of SCPP observations, including sounding cross sections, paired
soundings, and King Air data indicate that this perturbation
diminishes up the slope as indicated in Figure 3.1-4.
All of these patterns have been incorporated into the
transport module of the Guide model, where the basic input
is the Sheridan sounding winds. Note the dead layer in the
figure, where the normal component is nil. The depth of this
1-----__
I
12.0
I
90
I I
30 60
DISTANCE (km)
. 0
2-·- -­3-_
" ~ 4---
Fig. 3.1-4 Typical V component pattern.
3-6
layer is frequently one kilometer or more well ahead of the
front where area-wide and stable orographic clouds prevail,
and decreases to near zero post-frontally.
The input to Guide, for the orographic cloud's distribution
of liquid water, is a modified version of the pattern shown
in Figure 3.1-1. The modification is based upon spot observa­tions,
the pattern remaining the same with the amplitude adjusted
to fit the observations. In the absence of any observations
a cloud climatology would be used. It should be pointed out
that an orographic cloud can also be present when convection
exists, but is usually of limited depth, except in the case
of embedded convection. In late post-frontal cells it may
be non-existent.
Diffusion of particles on the scale of a seeding curtain
in theSCPP area have been studied in two ways: 1) in the
growth in width of a tracked seeding signature, where the
concentration of artificially produced small ice crystals
are measured over periods of up to an hour, and 2) the turbulence
spectra derived from measurement made by the King Air's horizontal
and vertical vanes. How these measurements can be used in
a practical way will now be discussed.
A curtain of nucleant is initiated by dropping a series
of pyrotechnic flares along a line. They are spaced out about
250 meters apart and each emits some 20 grams of AgI along
a one kilometer depth. Figure 3.1-5 shows a top view of the
starting configuration. In a second or so each nucleant trail
expands to several times the diameter of the flare, thereafter
dispersion is governed for several minutes by the law y2 = E t 3 ;
i.e., the width Y expands at the 3/2 power of time. E is
3-7
o
o
o
o
o
I' .... '",
I \
I I "\/~- ...... - -/" .......
I \
( \ \/ ) ....._....
....
/
I
I I,
"- .....
x
....
",
\,
/
/
./
START 5 MIN 10 MIN
Fig. 3.1-5 Typical flare expansion (circles) and
center dispersion (arrows).
the energy dissipation rate (measured by the MRI turbulence
meter). The expansion rate decreases when the size exceeds
the Lagrangian scale. This dispersion is depicted by the
circles that expand with time in Figure 3.1-5.
At the same time larger eddies disperse thE! centers of
the nucleant trails as indicated by the arrows in Figure 3.1-5.
The rectangles combine these dispersions and depict the resultant
curtain at different times. Note that within fLve minutes
the individual nucleant trails are starting to overlap.
Figure 3.1-6 shows a typical curtain width against time.
Beyond the first five minutes or so theory and observation
3-8
10
4 -Cl) AI
~ -- -CD CD
E-:r: 103
~
0-~
5 20 80
TI ME (minutes)
Fig. 3.1-6 Typical curtain spread.
indicate that the spread becomes proportional to t 1/ 2 • However,
it is both convenient and reasonable to use a linear rate
of spread shown by the dotted line A-A', that is an average
over the domain of interest to seeding. A faster initial
rate is used up to five minutes after start time. The one-sided
rate of spread varies from a few tenths of a meter per second
under very stable conditions to several meters per second
in convection.
3.1.1 Purpose and General Description of Guide.
The Guide model will ultimately be a numerical model providing
guidance for SCPP seeding operations. It will be used for
planning specific seeding operations so as properly to target
a desired area. It will also be used in the evaluation to
identify predicted areas of effect independently from any
response variates. In a sense, it can serve as a co-variate.
3-9
In its present state it serves to test various assumed seeding
modes under different assumed ambient conditions. These simulated
seeding runs provide insights into the bounds of seeding effects.
The general scheme of the model will now be describ~d.
The model assumes that a seeding "curtain" is produced
in aerial seeding, ei ther by means of droppable ~gI flares,
or dry ice (C02). A typical AgI seeding curtain is formed
by dropping twenty 20-gram pyrotechnic flares per mj.nute (about
100 g (km- 1 ). Curtain lengths employed vary with the situation,
but usually run about 10 km. The dry ice treatment is handled
in an analogous fashion. The depth to where complete evaporation
occurs (for normal size pellets) is about the same as the
burnout depth of AgI flares. The merging into a uniform concen­tration
of nucleant (really ice crystals) is possibly more
rapid.
The initial curtain then mixes with the environment at
rates that are input. This curtain mixing (or dispersion)
reduces concentrations of ice particles in the (~urtain, and
mixes in more liquid water from the environment so that growth
of the particles in the curtain can proceed. Bef~re terminal
velocities become appreciable, the curtain is advected by
the mountain air flow along a stream channel. When such a
curtain is produced and sampled it shows much higher ice crystal
concentration (ICC) and lower liquid water concentration (LWC)
than does its environment. This phenomenon has been called
the "signature". However, such a signature may contain artific­ially
produced fallout from a higher level. The term "signature",
and "curtain" as used herein, will refer only to nuclei or
particles so small that they move with the free air flow.
3-10
After some time, larger particles form and fall from
the signature, continuing to grow by deposition and riming
in the lower "feeder" cloud, until the ground is reached.
Some subcloud precipitation evaporation may occur if cloud
base is high above ground. This is most important in the
lee of the barrier.
Nucleation, ice growth, and precipitation are, of course,
normally occurring naturally at the time a signature is being
produced. However, as discussed in the previous section,
under seedable conditions a reservoir of liquid water accumulates
in the lower portion of an orographic updraft. In a stable
orographic cloud this reservoir would under natural conditions
completely evaporate on the lee side. Seeding taps this reservoir,
producing a detectable perturbation on top of the natural
processes. The complexities introduced when convection is
present will be discussed later.
With dry ice seeding, there is nearly a one step sequence
from signature to fallout. With AgI seeding the signature
is lifted by the orographic updraft to lower temperatures,
where new nuclei are activated. Accordingly, the signature
continues up to the crest, or at least to where dispersion
has reduced its particle concentrations so close to background
that it is undetectable. Fallout from the signature is continuous
along its full extent, once the initial stage is passed, provided
liquid water is available for growth. In the SCPP orographic
cloud setting growth within the signature is normally water
limited.
Falling particles change their terminal velocities as
they grow, and they encounter differing horizontal flows at
different levels. Therefore, their fallout trajectories are
3-11
complex. Guide works as a Lagrangian system for outputing
estimates of both the drift of the signature and of the fallout
trajectories.
The foregoing brief description of the modeled signature
and fallout processes indicates that Guide can be used to
estimate the fallout trajectories and ground irr.pact areas
(footprints) for differing seeding strategies under various
cloud type/wind flow regimes. The model is simple enough
for ultimate real time use. The basic elements of the computing
scheme are illustrated schematically in Figure 3.1-7. The
centerline of a signature (SIG) of width W, depth D, and length
(into the page) L is shown at three successive steps. It
expands as it moves, due to dispersion. At each step, one
fallout (FT) trajectory is computed. With C02 seeding there
would only be the initial nucleation and growth of ice particles
---
==
ORIGINAL
DROP
P2
Fig. 3.1-7 Schematic of signature and fallout steps.
3-12
up to position (1), with sUbsequent fallout of particles to
the ground beyond (1). With AgI a new set of nuclei are activated
(as the temperature falls) in going from position (1) to (2),
and these in turn start their fallout in going beyond (2).
The process repeats between (2) and (3) - etc. The signature
positions and trajectories in the vertical plane are computed
using the wind component normal to the barrier (U) and the
orographic upward component (W). In the horizontal plane
the component parallel to the barrier (V) as well as the U
component is used. The barrier wind flow module is used to
extrapolate from upwind sounding data.
Beyond the initial situation, stepwise growth of ice
particles on nuclei within the SIG depends considerably upon
the stepwise inward diffusion of ambient liquid water content
(LWC). The growth may be. water limited. In fallout (FT),
growth depends upon how the particles in the fallout plume
descend through the feeder cloud below. For accuracy in estimating
particle position, terminal velocities are needed, and this
is related to particle mass and habit.
Mass dependent, mass growth rate formulae were developed
as a convenient means for calculating mass growth during the
time step. There is also a dependence upon temperature in
the case of depositional growth, and upon liquid water content
in the case of riming. Depositional growth formulae were
constructed by adapting various curves of growth. Two different
\
temperature (crystal type) ranges are covered, providing two
options in the program.
The wind shear plays a major role in tilting the curtain.
Characteristic shear values of 10- 3 sec- 1 would result in
the top of a 1 km curtain projecting forward of the base by
3-13
3.6 km in an hour. A review of the geometry and co~sideration
of the fact that fallout from the upper part of the curtain
may make the lower part seem to spread downwind led to the
decision to apply the model separately to the upper and lower
halves of the curtain. The differences in the fallout trajectories
between upper and lower halves is enhanced by d:Lfferences
in terminal velocities. In the colder, upper ha,lf, growth
is inhibited by the excess of nuclei and the fallout trajectories
are flatter. In general, in the orographic case fallout particles
from different parts of the curtain are so sepa~ated that
they fall through a fresh water supply.
3.1.2 Some Guide Runs Illustrating Problems tn Orographic
Seeding. The two PET's, area wide and orographie, will now
be discussed. The principal problem in curtain seeding of
these two types is that diffusion is extremely sma:.l in them.
The curtain expands so slowly that overseeding :Ls the rule
for several hours, and in this time a curtain generated over
the foothi lIs would pass beyond the cres t. Thi::; si tua tion
dictates that seeding be started far upwind over thE~ Sacramento
Va lley, in order to produce the des ired ex tra fa,llou t over
the watershed. This procedure makes targetting morl~ difficul t.
A1 so, i twou 1 d bede sira b 1e t hat the cur t a i n bI~ in i t i ate d
in some cloud form, however thin. This may not be present
over the valley in the case of the strictly orographic cloud,
but would be present always in the area wide PET. Runs were
made using a LWC pattern with a peak concentration of 0.5
gm- 3 near the ground at the crest, and around 0.1 to 0.2 gm- 3
at the curtain level. This would fit with a natural orographic
precipitation efficiency of about 0.80. The dea.d layer was
100 mb thick.
Figure 3.1-8 is a vertical profile of the signature and
fallout from the lower half of a curtain having a point of
3-14
400
500
~ 600
E -7.7
w
0 700 z<
I-CI)
0
N
o 12 24 36 48 60 72 84 96 108 120
X DISTANCE (km)
Fig. 3.1-8 Signature (5), 5 fallout tracks (FT), and one
fallout phnne (FP) , lower half of curtain, orographic cloud.
origin at the 700 mb level, 100 km upwind from the starting
point that is shown in the figure. The opportunity to grow
precipitation size particles does not occur until the 4th
- 600 second step in this figure, and the first precipitation
is intercepted by the ground about 25 km into the section.
Precipitation continues to dribble out up to the crest, and
a little is carried beyond into the Tahoe valley. The total
fallout amounts to 18 acre-feet (AF) for this half curtain,
wi th a peak va I ue loca ted near the asterisk. The· reason the
particles fall almost vertically in the foothills is that
the normal wind component is very low at lower levels. An
average AW and 0 PET sounding was used.
3-15
Also shown is the fallout plume for a given time. This
is the synoptic picture of the plume of particles Wllich would
appear in a radar RHI section to be falling from thE~ signature
at this particular time. Note that this plume is very flat
near the ground as a result of shear effects. The width increases
toward the ground due to dispersion.
The temperature of the lower half of the curtain is only
-6.2°C at the zero point of the section but falls to -7.7°C
near the crest due to orographic lift. At these warm ~~mperatures,
the resulting precipitation particle concentrations (the compu­tations
are based upon the TB-1 temperature curve) are less
than ten per liter. Not all the available water is removed.
At a colder temperature, concentrations are more likely to
remove more of the available water. Figure 3.1-9 shows the
108 120
FT__-I>......
72 84 96
-12.2
24 36 48 60
400
500
SJ 600
E -10.2
wu
700
z
- 13
246.5° - 66.5° RADIAL
60 72 84
X DISTANCE (km>
Fig. 3.1-10 Horizontal section of signature (5), precipitation
track on ground (GT) , 3 fallout tracks (FT), and footprirts upper
and lower half midpoints (FP).
An acre-foot of water is 1.234 megatons of water. If
80 acre-feet fell over 2500 km2 (as above), then the J~ecipitation
would average .04 mm on this area. In a typical orographic
cloud the shear and terminal velocity effects spread seeded
water over an extraordinarily large area. If five non··overlapping
curtains could be laid in an hour the total mean pl'ecipitation
would be .20 mm according to the above calculations.
The Convective Case - Basic Concepts~ Convection
of some kind is present in about 85% of the cases sampled
by radar. It takes several different forms and these have
been categorized into the various PET types: In most of these
there is a mesoscale organiza tion •. In the cellula.r types (C1
and C2)' however, there is no such organization, although
they appear most prominently within an elevation range on
the upwind slope. It is these latter types that will be discussed
in the following.
3-18
The GUIDE model as presented so far might be adapted
to growth and fallout from a cell into an orographic cloud,
but something more needs consideration in order to put curtain
seeding within the cell itself into proper perspective. Figure
3.1-11 summarizes the present concept of the kinematics of
cells over the SCPP area. A stern zone contains an updraft
where LWC may approach adiabatic values. Above lies a top
zone where the inflow from the stem zone diverges laterally
and upward. There is a net mass flow into the top from the
stem zone (mean stem updraft Ws ) and outward from the top.
Since the top area exceeds the side area greatly, this flow
is best represented by a mean top updraft Wt which will be
much less than Ws as the top area is much greater than the
stern area. In the configuration shown in Figure 3.1-11 the
P
---.F3
F) =Cr Q9
~- -t--,....------+--'
t Ws
~
TOP -+- STEM
~
Fig. 3.1-11 Hushroom cell concept.
Typical Cell:
Top area =
Stem area =
Depth =
Duration =
100 km2
33 km2
1000 m
2- hour
3-19
whole cloud resembles a mushroom, however there may )e multiple
stems feeding a divided stream of air and liquid water up
into the top.
Simultaneous with the upward flows there is r~pid mixing
of environmental air with the top air (shown by dashed arrows
in the figure) which leads to a net flux of liquid water (F2)
and ice water (F4) out of the top. In addition, there is
the flux of liquid water (Q) from the stem into the top (F1),
and a conversion rate for liquid water into ice water (F3)
as a result of nucleation. New nuclei (capable of forming
ice embryos) are supplied continuously from the stEm, becoming
activated in the lower temperature of the top zone. Any ice
multiplication would be part of the total supply of ice embryos.
There is also a flux of ice water out of the top due to sedi­mentation
(P). The flux of ice F4 upward out of the top,
or downward as precipi ta tion, is proportional to thE! difference
between the mean top updraft W2 and the particle terminal
velocity.
The net mass flow out of the top is controlled by this
same mixing process, which very likely depends upon the air
mass wind shear near the top. With more shear, the mixing
is greater, and the total volume of the top (essentially the
top area) will be less as the mixing depletes the inflow from
the stem more rapidly.
The table in the legend of Figure 3.1-11 shows the typical
dimensions based upon recent plots of cells. Radar cells
for the 1976-77 and 1977-78 seasons combined showed larger
cells but this may have been the result of merging echoes
through a large depth.
3-20
The water balance equation in finite step form is:
#l =
fit
flQl =
fit
Fl - F2
k
MJ1k
L: Nk fit -
Ie
L: N. ~
.ok fit
k
L: (Wt - Vk) NIe r\
where Q is the LWC in the top, Q1 the IWC, Mk is the mass
for the kth particle set, Nk is its concentration, and Vk
its terminal velocity. The flux F2 is proportional to the
top LWC (Q) value, where the constant C2 equals C1, C1 being
the proportion of the top volume filled (or displaced) from
the stem during the time step t. The growth term for ice
particles (the third on the right in the first equation) is
summed over the K ice particle sets. The last term in the
second equation takes care of the losses due to flux of ice
F4 out the top and precipitation out the bottom. This term
has an upper limit equalling the total top ice water content
for a given set k. For steady state conditions the left side
of both equations is zero. A numerical simulation of the
developments from an initial state supports the concept that
a quasi-steady state does develop. The calculations were
based upon a stepwise solution to the equations, starting
with an assumed nucleation level, stem LWC value, and character­istic
dimensions. The GUIDE mass growth equations were used.
The computations were carried out in successive 300 second
steps. After quasi-steady state was achieved the smaller
ice particles, below about 300 m size, or 10- 5 . 5 grams mass,
had concentrations of about 60% of the active nuclei concentra­tion.
These are fairly representative of those measured by
the 2D-C probe. The remaining large ones, corresponding best
to 2D-P particles, had concentrations of around 15% of the
small ones. rhis relationship fits the values observed in
SCPP. It should be noted that in this numerical calculation
3-21
a spectrum of particle sizes develops. The spectrum is not
forced into any pre-conceived distribution. As new particles
are generated in each 300 second step, older ones are lost
either as precipitation out of the base or evaporation out
of the top.
The results with respect to 2 D-C concentrations (arbitrarily
60% of the input nuclei concentration) and quasi-steady state
liquid water content are shown in Figure 3.1-1:!. Each of
the numbered curves is for a different cloud having the typical
dimensions shown in the Figure 3.1-11 legend and having stem
LWC (plus condensation in top) ranging from a high value of
2.5 g m- 3 to a low value of 0.5 g m- 3 • The stem updraft was
2 ms- 1 • The numbers lying along each line are the calculated
steady state precipitation rates in mm hr- 1 •
Looking at the line marked by stem LWC and Top Cond. =
2.5 g m- 3 at its base, we see that if the measured :~ D-C concen­trations
were .06 per liter then the model predi~ted steady
state precipitation rate would be .07 mm hr- 1 ind the LWC
would be .2 g m- 3 • On the other nand, if the mea.sured 2 D-C
concentrations were .6 i-l (due to a colder cloud, or more ice
multiplication, or artificial nuclei), then the steady state
precipitation rate would be .20 mm hr- 1 , the LWC .2 g m- 3 •
At a still higher 2 D-C concentration of 6 i- 1 the precipitation
rate would be .29 mm hr- 1 , the LWC .2 g m- 3 • Now if the 2 D-C
concentrations were 60 i- 1 , then the precipitation would be
reduced to .21 mm hr- 1 , and the LWC to < .02 g m- 3 • A still
higher 2 D-C concentration of 600 i- 1 essentiall~ eliminates
both precipitation and LWC, the steady state cl)ud being a
swarm of small ice crystals.
3-22
3
ZERO
\
2
-u
zou
UIA
N
oLL. 0
C) o
..J
.21
.04
.02
.5
.29
.20
.06 .07 .'- STEM LWC
..!.:.Q. b2.~ + TOP CONDo